Nearly anyone can bolt on a new starter... and most mechanics who wish to make money do just this. (They get paid to bolt it on and also get their 'cut' on the starter cost)
I have fixed many 'bad' starters. (I do not 'rebuild'...no repainting.)
The starters I have fixed were from Snowmobile, lawnmower, car, truck, JohnDeere bulldozer... and even my Kubota tractor.
In most cases, it is as simple as disassemble. clean, lubricate and resemble.
Sometimes I have to fabricate a replacement part, make a spring, add some silver-solder to contact-points, resolder a wire....etc
One was simply a lack of lubrication.... it would spin fine with no load... but as soon as it had the load on it... the bushings would have so much friction that the starter would stop spinning. A good cleaning and lube of the bushings fixed that issue.
In many cases, I use dremel-tool with fiberglass brush to clean the area where the brushes make contact. (commutator)
I have NEVER encountered a starter which could not be fixed.
In some rare cases, the gear on the starter may need to be replaced if the teeth are worn down.