Not too sure without seeing a snapshot, from experience most decoupling caps i've seen on amiga hardware are 0.1uF ceramic (usually printed as 104 or 104K on the cap itself) for the ones between vrails and gnd. They can be a major pain to test without a decent multi/capmeter, and generally its worth replacing the lot since they're worh about $0.50AUD a pair.
I prefer to use monolythic caps for decoupling my microcontroller based apps (up to 20MHz), they'll set you back $0.50AUD a piece - I havent found any difference performance wise bewteen monolythic and tantalum when used purely for decoupling; both worked equally well.
Also, caps used between oscillators (crystals) and ground are usually 33pF afaik, 22pF should also be fine; there are generally only two of these per oscillator, one between OSCIN and GND, and one between OSCOUT and GND
Hope this helps
/Cp
