When I designed my new game engine, I used the old one as a base and tried to keep as many essentials in place while fixing any screwups and bolting new features onto the hub so it could serve as a drop in replacement scripting wise.
Or such was the idea.
In practice, the drop-in worked, but with horribly messed up game engine timing issues, and it hit me as to why:
RPG Maker's events are synced to any code designated to be active when first loaded, and since I changed up a lot of existing settings when the events expected different timings according to the scripts they used, this led to choppy framerates everywhere, so I'm gradually importing the new engine settings while fixing goofs as I go along.
I also have another issue: The new portraits in place of faces works fine, but results in some text misalignment in text boxes, currently looking into ways to compensate for this.
This has also forced me to disable any pixel movement in place of stock movement until the timing issues are fixed, as it plays merry hell with the framerate and screen refresh settings.
Progress is proceeding somewhat slowly due to these snags, but it is proceeding.