because i believe with enough code-tinkering and some skill in programming, every kind of games can be converted into any engine
This is a very brave assumption, and very likely won’t hold true. Different engines have different assumptions and expectations. I doubt it’d be literally impossible, but it’d be less of a conversion and tinkering task, and more of a “literally rewrite it from scratch” type of task.
and thus will increase the game’s quality
That’s not how it works at all. There’s rarely such a thing as a ‘better’ engine for everything, but rather a best engine for the situation. By all accounts, the Frostbite engine used for Battlefield is a great engine for shooters, yet Bioware struggled hard to get it to work for their RPG shooter hybrid Mass Effect: Andromeda, as the engine wasn’t natively suited for RPG mechanics and dialog tree’s.
As far as Unreal or Dunia goes, they’re not going to be very well equipped for CDDA’s needs. The effort spent making it work as CDDA needs it to would be better spent either making a brand new game, inspired by CDDA to take advantage of the engine, or just continuing work on the existing engine.
I’m not aware of any hard limiters in the current engine, most of the work is limited by the volunteer time contributed that meets the standards of the project. I don’t see how such a migration would be worth the effort.