They should be commercial screws, probably metric. You would need to find out the pitch (with a gauge), diameter, length and head type from one of the other screws. Usually all Boss products share parts, so the parts could be scavenged from an old, useless pedal. The missing knob, too. As for the internals and electronics, take it to a tech. Amp techs sometimes accept checking pedals.
Answers & Comments
They should be commercial screws, probably metric. You would need to find out the pitch (with a gauge), diameter, length and head type from one of the other screws. Usually all Boss products share parts, so the parts could be scavenged from an old, useless pedal. The missing knob, too. As for the internals and electronics, take it to a tech. Amp techs sometimes accept checking pedals.