More context is a good thing on Trump's bluster, as GM is only 10,000 cars away from their 200,000 vehicle cap on EV tax credits anyway. The credit will go from $7500 to $3750 for the 2nd quarter after hitting that target, then down to $1875 for another 3 months before hitting zero.
https://www.edmunds.com/fuel-economy...x-credits.html There are efforts afoot to extend that credit, which is the main thing he could affect.
Now back to motors for conversions. Analogous to rotor tip speed with helicopters, the speed of the commutator under the brushes is a limiting factor on series wound brushed motors, which is proportional to the motor diameter. So a larger motor will have more starting torque, but a lower top RPM. Since starting torque is rarely an issue in EV conversions but highway passing power is, a smaller motor, or especially multiple small motors can be the best solution, as they can reach higher RPMs. Efficiency will not change appreciably between say a 6" and a 9" series motor.
My 9" series motor has externally wired fields that can be switched from series to parallel. This provides maximum starting torque in series, and then further down the dragstrip, switching to parallel provides more power at high RPMs.