A mid drive kit is more expensive than the hub motor, at $400-600, Since you want to use your 36V battery, that will probably limit you to the less powerful kits. I think the biggest obstacle is that you have a single speed trike, and that's going to be hard on the mid motor. You want gears on small mid drive, like we used to want stick shifts on small car motors, until automatic transmissions improved. If you had a derailleur model, a mid drive could work,