Then, to be extremely frank, it sounds like the game is a technical failure and should be abandoned. Trying to fix an older game such as this seems like an uphill climb. Better to greenfield it and start from scratch. The original engine sounds like it needs a rewrite anyways.

Getting it running on an existing engine (UE4 can do 2d fairly well, for instance) seems like a better investment of resources. What they're building is a solved problem. It was a valiant effort to do it on their own, especially in a time when that was the only way. They could reap the benefits of all those learnings in the backend and instead focus efforts on tweaking gameplay, inventing new forms of play, and smoothing out rough edges in the UI or visuals.