It's been many many years ago(like 10y maybe), but I remember an article describing the pinouts and differences of the amiga roms and standard eproms. It was possible to use standard eproms if an "adapter socket" was built and used along with a standard eprom. Some hotbot.com, google.com or aminet searching might find the old info. I used to make custom C64/128 proms the same way for me and a few friends. Kind of nice to have your computer boot screen display your own name/address/message when you turned it on. My memory isn't fantastic going back that far so please be kind if my info is faulty. And don't forget about the legal traps. I'm sure some one will be along to fill you in on the copyright issues. But I think if you're only interested in modifying your own system, you should be safe.
Plaz