Controllers are cheap.
https://kellycontroller.com/controllers/brushless/
You want a small amount of net regen, so the pedal needs to be pressed a good amount before electric assist kicks in and otherwise it's charging the battery to provide power to the car. One way to get this is have some simple circuit to change the electric throttle signal in accordance with battery voltage (lower voltage, reduce throttle).
You can have a reduced capacity 12V battery or not to save space/weight for the hybrid battery. You want a DC-DC converter able to supply 12V power. 13.5V combined with a LFP battery is what I would go with.
The hardest thing other than mounting the motor or converting the alternator is honestly going to be the tensioner. You'll have to do a lot of measuring and adjusting to create a mount for the tensioner and get it aligned to the belt.