From
SxROM series boardsSBROM: 64 KiB PRG, 16-64 KiB CHR ROM, no WRAM
SNROM: 128-256 KiB PRG, 8 KiB CHR (almost always CHR RAM), WRAM with optional battery
This would require the usual rewiring for PRG ROM but a bit more extensive to switch the CHR side from using RAM to using ROM. If you need to use a donor, I imagine a donor using
SFROM or
SLROM might be easier.