I'd first be checking whether the physical contact-switch is the problem. They typically contain a tiny spring-metal dome that can get deformed or broken in some circumstances, or the action can be blocked by a speck of dirt of some kind. First check the on-off operation with the continuity tester on your multimeter, and if it fails look for a mechanical reason. If it needs to be replaced, the actual switch mechanism is quite generic, but it is also possible to wire in a non-standard contact switch if you have to.

IIRC there is also an isolation capacitor near that switch, which I would look at next if the switch appears to operate correctly.

Deeper than that... :-/

Rick