AFAIK most StarMax systems that were released (a prototype exists of a CHRP StarMax model) are based on the Tanzania / LPX-40 design, which is mostly a traditional PCI PowerMac[1], albeit with oddities like support for PC style floppy drives. PS/2 is handled by the CudaLite microcontroller which presents it to the OS as ADB devices for example. I've not heard of a version with ISA slots, although I assume you could just have a PCI to ISA bridge chip, even if MacOS presumably wouldn't do anything with it.

[1] https://cdn.preterhuman.net/texts/computing/apple_hardware_d...