A couple of years ago I decided to start taking a part Sonic Drift. I got around to adding a hacking guide on Sonic Retro's wiki, which you can find in the link below. I've got the urge to dive back into hacking the game again so when I get some more free time I am going to post my findings in this thread. I still need to figure out how the track palette works, and I think the best way I can tackle that is by possibly taking a look at how Outrun on the Game Gear handles the tracks. I would love to also figure out how to fully modify the track layouts too. Here's the wiki page.
https://info.sonicretro.org/SCHG:Sonic_Drift