The author of AmiMasterGear did AmiPCEngine and also AmiMSX &
AmiGameBoy. They are all very good but the PCEngine one gave him
the most trouble it seems. After all, the 8-Bit PC-Engine had a
version of Streetfighter 2 the equivalent of the Genesis/Megadrive so
there's got to be complex custom chips in there.
What I'd like to see for OS4 is a good Genesis/Megadrive emulator,
that would be a lot more nostalgic for the majority of westerners.