I restored one when I was about 12. From what I remember, cleaning was the big challenge. The one my grandfather brought home had been exposed to dust, cigarettes, and sugary alcoholic beverages. That muck prevented the tiny mechanical bits from working correctly. I had to clean each target to get it working. Be sure to clean the balls and raceways or that muck will just get transferred to the mechanical bits.
I also recall having to fix some minor wiring faults. A multimeter (test for conductivity / resistance) was needed.
The machine did not need any replacement parts.
It was similar (but simpler) to rebuilding an automobile engine. Each bolt had a place. Each part had a place. Make certain each part finds its way home. Little plastic bags, a small pad of paper, and water resistant ink are very handy.
Bear in mind DMS has the tools needed fabricate nearly anything broken / missing.