Like CodePoet said, just rip them all out and replace them.
Just checked on Maplin, they have them (ceramic disc 0.1uF 16v) for 5p each.
Did this 501 ever work in your A500? You say all the traces are intact, and the 501 is a simple design so I'm just wondering if the 501 could be OK and the A500 at fault.
Maybe a previous owner tried the hack to get the 501 as chip ram or something?
