Then your repairman will have to give us the board numbers. Kustom amp boards have type numbers on them, PC5033 or PC108 or whatever. We need those PC numbers, as that is how the circuits are identified. The "schematic" for an individual amp model is really just a wiring diagram for the connections between boards.
There is a lot that can be done without schematics. We can usually verify power supply is present and clean. And if we have power, we can usually determine which section has the problem.
"Stopped working" could mean anything. It would be helpful to know whether that means the whole thing is dead and dark, or if the sound no longer comes out the speaker, or if it still amplifies but sounds like crap.
And it helps to know if it was failing for a while or if it worked OK and then just stopped instantly.
ON an amp that age, all the small electrolytic caps in the signal path are suspect, and for the very small cost they represent, they probably ought to be changed, and that usually fixes half the old Kustom amps I see.