The starter for the early B21/23 engines have clearance holes in the mounting tabs. The bolts go through the bellhousing, through the starter mounting flanges, and are secured with nuts.

The early B230 engines have similar starter motors to the B21/23 but the holes in the mounting flanges are threaded -- no nuts required.

The later B230 starter motors use a lighter, stronger, faster Japanese starter motor that incorporates an internal reduction gear to produce the torque.

I believe that any of the three designs can be substituted for the other, but you might want to be cautious about the electrical connections at the solenoid. It would be best to get the right starter for your car so connectors and bolts all fit properly.
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Don Foster (near Cape Cod, MA)