Water-based solvents like Fast Orange in a recirculating bath parts washer are pretty insulation-safe, although I've cleaned up dozens, if not hundreds of generator and starter housings with a quick dip and scrub in a bucket of gasoline back in the bad old days when I was 10 feet tall and bulletproof, without damaging the insulation of the coils. Electrical insulating varnish is pretty tough stuff. A quick work-over with a heat gun or a little while in a warm oven dries the housing out while you're doing other stuff like turning the commutator, undercutting mica, changing bushings, etc. Unless we were dealing with a very scarce item, it was a lot simpler and more profitable to use good-quality rebuilt starters and generators, but that was also back in the dark ages when parts were made to be rebuilt and enough of them were on the road to make it worthwhile to do so.
