You can compile 68k assembly to object code and include this code as binary blob in your PPC native build. Or even in 68k native build if you want to. I did that to Scout and FileMaster long ago and I just called 68k code from PPC code using the built-in 68k emulator.

However, inline asm is different case and in this case it would be better replace inline asm with native code. You find some useful macros from hardware/byteswap.h header file.

So try to replace inline asm with native code. Perhaps use macros to conditionally use 68k inline asm or native code or MorphOS macros that do the same.