the CPU / FPU as long as they are PGA and rated close to 50MHz then whoo
(I woudl consider some active cooling since its in a confined space, something like a small slimline chipset cooler
now the Crystal needs to be 14pin DIL package, although you could easly modify most other 50MHz OSC that you can get from AmigaKit, infact Matt might have some of the 14pin DILL packages in store my friend.
alas there nothing worth anything on eBay for your project that I could find at the moment my friend.
I will need your address for the ROM chip though