I've had pretty good luck with changing bearings in Lucas generators, although finding them can be a problem. ISTR that last time (which was quite a few years ago), I had to buy a longer rear bushing and cut it down. It'll probably be ready for new brushes as well.
Sorry, I'm not familiar with any GM DC generator that makes a drop-in replacement. The one on my 62 Chevy was quite a bit larger physically, and took a different regulator.
But lots of folks do various alternator conversions, I had a 60 amp Ford on my old TR3A; and at some point I'm probably going to try a Suzuki unit on the TR3.
PS, many years ago I limped home by cutting a strip out of a McDonald's drink cup and using it to fill the gap in the rear bearing. The bearing had gotten so bad that the armature was dragging on the pole pieces and making a terrible racket. Not a long-term fix, but it did get us home!