Yeah Gav it is the 6800 if yu have a spare that would be great
I'm happy for someone to tell me I'm wrong, but I don't think a 6800 will be ok for a sys 6 cpu board? A 6800 needs an external clock generator, this is more than just the crystal.
You need a 6802 or 6808 and then there are a few jumpers to set depending on which of these you use as one has onboard memory.
Processors do fail, but I wouldn't think it's that frequent - have you stuck a scope or logic probe on a few pins, any of the address lines, the VMA or R/W would be fair ones to examine.
Any sign of activity on these pins would suggest to me the processor is ok and have me looking elsewhere. Do you have the WMS test rom?