I replaced the foam in a Lisa keyboard not too long ago. I did all of the keys and it was quite time consuming, but in the end they all worked fine and it was worth it.

I used some 1/8" thick foam mounting squares from a craft shop and trimmed them to fit. Ideally, if you could find something that was precut to the approximate size of the disks, that would help quite a bit.

The old foam just disintegrated while I worked with it. I used an Exacto knife to scrape as much of the old foam off of the mylar and plastic disks. It isn't necessary to get them absolutely clean.

Good luck! (Though it sounds like your power supply is a more pressing issue.)