[DonRotolo]

A typical starter motor can deliver 1.5 to 2 kW of power, briefly. That is quite a bit more than a CIM. At those power levels, cooling is an issue. A very reduced load (still tons more than a CIM) would reduce that a lot.

Starter motors are not designed to run backwards, the brushes are optimized fir one direction only. They are also not designed to run for long periods of time, maybe a lifetime of a few hundred hours.

But they ARE cheap.

You will want to disable the solenoid that extends the pinion. An easy interface to the rest of the world is a sleeve that fits over the pinion gear teeth, with an output shaft to fit what you want. Make it on a lathe to ensure it is concentric.