They need to just bite the bullet and move the whole thing to another engine. Godot is free, Game Maker and Construct are cheap. Right now Something Unlimited is basically a Looney Tunes gag where a car drives out a garage and everything falls off at once leaving Daffy Duck or whoever just holding a steering wheel.
Unity is fine, their problem is that it was never meant for games to be produced this long, you can't update the version so SU is stuck in an ancient version, and with an ungodly amount of spaghetti coding.
While it is true that pretty much any revamp made by an actual coder in any engine would be better than what SU has, I don't think that is happening any time soon. After multiple people working on SU Rebuild plus that fiasco with the Akanoes Scooby Doo game, I would say their problems go beyond just technical difficulties, being burned once is part of the business, twice is bad luck, when everyone you hire doesn't work out maybe the problem is not with these people.
Being realistic, the only fix for the SU's engine problems is if OhWee and xMendonza ever finish their Ren'py port and receive a big fat check to sell it to Sunset. Even then, chances are pretty low that it would work well under Sunset's management.