Edit: seems I didn't read everything.
Yeah, go for it. The contact foil usually isn't the culprit, but the elastomers under the key stems. You can swap them from the A500 keyboard or buy a PC Mitsumi keyboard and get them from there.
All you need to do with the foil is use a moist cloth and wipe off all the dust you can see. Then use TP or some other household tissue and dry it up very carefully so that it doesn't start oxidising (very very important, the metal bits are very thin and oxidisation eats through them very fast)
Also, my trick for revitalising the elastomers: crack open the keyboard, press down on an intermittent key to expose the elastomer, then just wipe the elastomer with a piece of office paper from your printer.. Mind the sharp paper edges, as they can cut the elastomers off.
A few wipes will leave some of the elastomer's hardened surface on the paper and expose the good bit underneath. Don't wear them down too much!
This works with most Mitsumi keyboards, be it Amiga, CBM 8-bit, or PC. :-)
