I have an old (circa 1980) microwave, and eventually the timer wore out. While i could fit another in there, I rather did a simpler mod and put a mains rated pushbutton switch and a industrial timer module, so that I had a 2 minute timer instead, and a single push gives you 2 minutes of operation. Open door and it turns off and resets the timer. I did replace the mains fuse once when it blew from the inside lamp failing. Replaced the lamp a few times, and other than that it is still the same parts as new, and as it has no turntable but a air driven stirring vane in the waveguide feed area it does not wear the cavity.
Amazing a KIC microwave still works nearly 40 years on, and is in regular use. Those switches can be a pain when the door wears, as they now use a single plastic latch mechanism that holds the switches and it can deform with time and heat.
And yes, every single microwave has a schematic of the wiring inside the cover, complete with the part numbers of the switches and fuses, and with the part numbers for all the things like magnetron, transformer, capacitor and diode. You also in some get a protection diode, basically a 3kV avalanche diode there in case the magnetron fails, to break down with the overvoltage and short out, blowing the mains fuse. A pox though on manufacturers that use a single security screw on the cover, the rest being regular pan head self tapping screws. Takes an extra minute to open the cover every time, and i generally just replace it with a regular generic microwave screw.