The thing about HDL-code is that once you have it, the supply is secured. Speed might suffer but you can run things. And as time progresses FPGA gets faster and cheaper while chips might be EOL'd. Though currently FPGA implementation would not be cost effective.
How is compatibility between classic PPC accelerators and currently manufacturered PPC chips?
(I recall the only available m68k has an incompatible instruction "MOVE sr,")
I think you confuse PPC and coldfire... Not to mention I have no idea what you are trying to achieve... A PPC on an FPGA would be painfully slow
