I take it your generators make use of the Cummins 4BT and 6BT engines. If it's broken at 3AM then it's broken, difficult to have all the right parts in stock. If its some thing small like a fuel hose, radiator cap, relay or AVR that is not a problem. If its anything bigger like diesel lift pump, water pump or even the fanbelt and tensioner no can do. Imagine the rack in the injector pump gets stuck, you need to remove the complete injector pump and take it in. The amount of electrical parts that can break are vast as in: starter, speed controller, contactors, auxiliary contacts, AMF / ATS controller and circuit breakers.
Generators do break down, the best you can do is to have a good preventive maintenance plan in place EG check them thoroughly every month and test them. Once a year put them on a load bench and hammer them for 1 hour on max load, this way you will ensure that the cooling pack works correctly and the motor does not glaze up and loose performance. I know of a 6BT that has done over 12000 hours and has not missed a beat, it gets serviced religiously ever 250 Hours.
Send me a PM I'm sure I can be of assistance, been doing it for over 10 years.
