A4000/A2000 keyboards are compatible. The only difference is the plug, and adapters can be bought at any computer store. I think AmigaKit even has them, but I don't know if they are the direction you need.
I seem to recall the C64 P/S was a little weak for the job, and in my experience, A500 P/S's are very unreliable (most likely due to their age by now). Actually, if I were you I'd just build a power supply from peecee parts. The connector is easy to get. RadioSchack even sells them.
