I remember back in the early 60s and the owner of the shop I worked in used to go to the junkyard, and buy a pickup load of "core return" starters and generators, and we would rebuild them for resale in his shop. In those days, several sizes and types would fit almost every American car. We had an assembly line set up, and the two of us would do about 150 units in a very short time. In the 5 years or so that I worked there, I really don't remember any that came back as not working. This is not so today when a (and I love this word,) "reman" unit has an immediate failure rate of at least 50% or so. (Chinese)
Anyway, (whew!) it's not hard to do a starter or generator, and even the 1st generation alternators. Getting quality parts to do so may be the hardest thing. Any old Motors or equivalent shop manual will give detailed instructions. The important thing is to do the entire unit, not just clean up and brushes. The armature MUST be turned, the bearings MUST be replaced, along with the brushes, and checking the fields and armature for shorts.
The attraction for me at least, in a mini starter is the compact light package, the increased spin, and the absence of that old wheezy after run that pulls the drive back on the old starters. They also require much less current to operate. BUT, the attraction for the original still remains for most people.