Got the Angel Island map from Sonic Adventure (1998) running natively in the Sonic R (1997) engine this weekend. Same minimap, same leaderboard, completely different terrain. You can race the loop.
Texture remapping was a nightmare, and there's still some clipping where the falloff fights the mesh segmentation, but it's racing. Had to rework the engine's per-track memory limits to swallow the higher-poly Sonic Adventure assets, which was its own little adventure.
A Sonic Adventure import turned out to be roughly 80% of the same problem as authoring a track from scratch. Which means the from-scratch goal is closer than I thought. Full writeup over on the Sonic R Mod Maxxing page.