Just make a shaft to replace the shifty gear reducing bits of the transmission and then bolt your electric motor to the now gear-less transmission. It should be pretty easy to standardize a coupling and mounting bolt pattern for the motor and then just bolt on whatever transmissions shaft and bell-housing adapter you'd need for the particular model.

I'm glossing over the details but you'd need about half a dozen precision machined features and like three or four parts that are specific to a particular ICE engine/transmission application. Dimensions are such that you could probably buy cheap axles for a common application and then have them machined down to make your part. Parts cost should wind up under $200ea unless you need something expensive to machine (like blind female splines).