The whole thing that makes cryptocurrencies work now is the un-cloneability of the amount of work done (which is why the 51% problem exists in the first place).
The only good way of doing it would be through a cartel with strict enforcement of hashrates etc.. And welcome to the financial banking system...
IMHO cloneablitiy IS the problem, PoW is just a (very, very unfortunate) solution to that. PoS (stake as in possession, not pledging) is IMHO inherently impossible, so PoW is all we got, now and for ever...
Can you elaborate more on what "uncloneability of digital assets" means exactly and why it's a pre-requisite of proof-of-stake? Happy to take some links if there are any that explains this well.
In the real world cloning money (historically precious metals) is really really hard (and btw. much easier with paper money that the metals that have a basket of physical attributes that cannot be emulated at once).
Digital bits on the other hand are indefenately cloneable (by design, that is one of the base properties of "digital").
Just for you to see my text here, along the way somewhere around 20 perfect copies were made by various routers (i.e. computers)..