Other things to consider, check the grounding cable from the engine to the airframe and the power wire to the rear engine starter. Sometimes reinstalling these after an engine change gets missed.
You may not be getting a full 24v to the starter because of a weak contactor or corrosion in the starter wire, or the grounding cable may have a voltage drop due to poor connections.
If the engine is not spinning fast enough, the impulse couplings may not be engaging.
Check the resistance from the starter motor mounting stud to the rear cowling mount flange, and check the voltage to the cable stud on the starter motor. It shouldn't be a volt or two below battery voltage.
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