The crank speed sensor will feed an engine ECU, which you will not have any more, so that sensor doesn't matter. Whatever you get to transmit an "engine speed" on CAN (presumably some programmable box with a CAN interface) does need an input (unless it always generates a fixed speed), but it doesn't matter whether or not the sensor is similar to that used with the engine.Can the original crank speed sensor be used??
Also, wouldn't the motor controller produce a motor speed signal of some sort? And more importantly, what do you want the "engine speed" to be, to get the desired vehicle behaviour?