Local automotive electric motor repair shop is probably your best bet. Price, unknown. Last starter that I had a shop repair cost me $60-, but that was years ago.
Suggest disassembling (make a sketch of the internal wiring). Find out what is the problem(s). It may not be an expensive repair because brushes, bearing and bushing, can be obtained from your local auto parts store. Field windings can also be purchased.
Local library, reference section, look up starter or generator repairs.
Question or a statement: Battery condition, solenoid, regulator, faults show up as a non functional starter/generator.