It’s worth having a look at the BBC Micro’s facilities for a second processor
(aka The Tube) - included a larger faster 6502 as one option, or z80 and x86 for
compatibility, or ns32k for performance.
There’s a BBC Micro emulator called B-em which also emulates the Tube,
and in fact there’s even a National 32000 “co-emulator” which will
run Acorn’s PanOS operating system for the 32000, their Fortran compiler,
and other tools.