I did a bit of digging into why this didn't work...

First of all, it has a few instances of instruction 0xFFFF in the code, which triggers the fatal "illegal GRP5:7" error. Ignoring it isn't enough, as the whole thing runs amazingly slowly (as previously reported).

The solution is just to trigger an illegal opcode exception, as the game uses it for some strange reason! The result is a working game! 😁

i.e. just stick this in the "default:" at the end of prefix_none.h and comment out the rest.

CPU_Exception(6,0);
continue;