If you try to equip an An-124 with two 50kgf engines you will need to design a new wing.
From what I can tell, the PD-35 is being driven because the PAK DA will likely use an engine in that power range, though it might be an internal engine more like a turbofan in a fighter aircraft than a high bypass airline engine type.
From that PD-35 they plan to make a two engine and a four engine version, where the two engine model is smaller and lighter and much cheaper than the An-124 to operate... an Il-106 class aircraft that essentially would be replacing the AN-22 which was a popular and useful aircraft, but with only two reliable and fuel efficient jet engines would also be much cheaper to operate than the bigger not so reliable and not so fuel efficient An-124.
The pay off would be that they would want Il-106 aircraft to replace both the AN-22 and An-124 aircraft but that creates the problem that the Il-106 that is smaller and lighter and more efficient might stretch to 120 ton payloads using inflight refuelling, but it wont carry the heavy payloads the 150 ton capacity An-124s could carry... which is why the Slon is being considered too... with four PD-35 engines it almost has as much thrust as the An-225 so a 180 ton payload should be easy and with the space industry expanding having an H tail aircraft able to carry outsized loads on it back would be useful too.
They wont need as many Slons as they have An-124s but they will need a lot more Il-106s than they had An-22s so they will end up using a lot of these engines.
The modular nature means scaling them up or down in thrust performance is supposed to be rather easy and straight forward so 24 ton thrust models for An-124s would mean being able to continue to use them much longer, improved performance and fuel economy and the ability to poach an export market and domestic civilian operators of the aircraft too.
It also creates a future potential for a twin engined Il-476 weight aircraft, while the twin engined Il-276 could have two PD-14 engines to replace the An-12.
There is little point in developing a new modular family of engines if you are not going to make them in different power ranges for different aircraft.
Obviously a 50 ton thrust engine would be for a large airliner or air transport plane or could be used for a powerful ship... military or civilian... or even a large hovercraft of maritime patrol aircraft... or power station...
You are changing the weight loading on the wing, how it's distributed, and how the air flows around it (remember the wing bends in flight). It's not going to work with the old. You might be able to get away with equipping the An-124 with four PD-24 engines and just need to update the flight control system. The design calcs would show for sure.
Designing planes and wings is what they do, there is no point in putting new improved engines on an existing aircraft and not doing anything to improve that aircraft.
The point is that the Il-106 was designed for specific engines but when those engines were cancelled they would hardly continue the project based on Ukrainian or foreign engines... they would have decided what engines they would use and plan for different wing and engine pod and engine pylon arrangements to suit each engine option... as I said they have been talking about the PD-35 for quite some time, and it is not an accident the PD-24 is similar in power to the engines it would be replacing on a Russian An-124.
The Slon was always expected to be a larger aircraft with better capacity than the An-124 based not only on their experience with that aircraft but also planning knowledge of new vehicles and equipment the Russian Army will want to be transporting around the place.
These plans of course can change... the withdrawal of the US from the INF treaty means much heavier truck vehicles with much heavier multistage IRBMs would become an option for air transport, but I suspect a 180 ton capacity Slon should be able to handle most options as it could probably carry ICBM level vehicles and missiles so IRBMs should be no problem either and scramjet powered hypersonic missiles will be much lighter and faster too.
Now it is true from a technical standpoint that Russia can redesign a wing for it. But that may not be the best option for resource (engineering and manufacturing) allocation
Pretty sure they could calculate which is the better options in terms of life cycle costs as to whether a much more powerful engine in a twin mount could work out cheaper than the current arrangement with PD-24s, but as far as I know the focus for PD-50s would be heavy airliners like the Il-96 or Il-86 with two engines to reduce maintenance costs and flight efficiency with lower drag.
For the PD-24 you can re-engine the An-124. But Russia can refurbish the D-18T and so that might not be the most cost effective approach. And there won't be aby more of the aircraft made. In order to make it cost effective, they need to produce the engine in volume. They could do this with a follow on design (you suggest an Il-106 but I am very skeptical that Ilyushin can pick up another design right now). But there is clearly nothing within the next five years.
The PD-24 would eliminate Ukrainian parts from the An-124 design for the Russians which would be useful and could be an option for a twin engined Il-476... two PD-24s would give 48 tons of thrust, while the PD-14s they are going to use for the upgraded Il-476 would give 56 tons thrust, the original Il-76 would have four 12 ton thrust engines which is also 48 tons thrust but with two engines better fuel efficiency and lower maintenance for a lower operational cost for lighter 40-50 ton payload operations it might be an option, and of course with four PD-14s on the Il-476 and two PD-14s for the Il-276s to replace the An-12s, perhaps two PD-18s for the Il-276 might give better range and payload performance options.
The PD-35 was for the PAK DA and Il-106 replacement of the An-22, using twin engine arrangements while four engine use on the Slon to replace the An-124... but who knows.
With PD-50 in the PAK DA for a bigger more capable aircraft and PD-24s to keep using the An-124s for much longer, and the Il-96 as a more efficient twin jet...
It all depends on the performance and capacity to produce engines and the efficiency of mass or small production batches.
For the PD-50, you can make a further stretch of the Il-96 - and build a whopping ten of them. Not cost effective at all. And there aren't any other possibilities in the near future.
But that is the question... with modular engine designs do you need to produce them in enormous numbers to make them cost effective or does their advanced design and low fuel burn and excellent reliability make them good cost effective engines anyway.
They may have projects we are not yet aware of, or they may simply see it as a future trend or market niche they see interesting to cover or where they want to compete with the West.
The 50 ton engine might be for a power station or ship... and the reliability and fuel efficiency and modular engine design might make it useful and viable low risk design.