No hblank interrupts? Game boy has them to make this sort of thing relatively easy. Hblank interrupts is also how "mode7" style pseudo 3d for a Mario kart style game can be made (on GBA or SNES).
https://en.wikipedia.org/wiki/Memory_management_controller#M...
If so, what approach are you taking for the toolchain? Keeping the existing version that had been used/modified? Updating to newer tools (and code changes that may go along with that)?
Can definitely see wanting to keep the compiler and code in a known state. Though modern SDCC generates code that's probably 50% faster in some cases and with 90% less bugs than versions from the early 2000s.
Lots of great Game Boy releases have been coming out.
Edit: add a little more.