I'm not sure I agree. You would need 24 LFP prismatic EVE cells (Which are EV rated) to get to ~20kWh (72v at ~300ah). They weigh about 5.5kg/cell. That's nearly 300lbs in cells alone (not accounting for wiring/support/cooling). I would pretty easily see total weight hitting 500lbs (or more).

You can do it more efficiently with different chemistries, and you can scale down your storage for less range - but 500lbs seems pretty reasonable for a retrofit.

Ex - your own linked kit doesn't include any storage, and the motor alone weighs 120lbs. There is NO WAY you're getting away with 150 lbs when the motor alone eats 120lbs.

Lead acid... come on... a 100ah 12v lead acid (1.2kWh - of which only .6kWh is really usable because of chemistry) weighs 70lbs on a good day. A normal EV will do 3-4/kwh. 5kWh of usable charge (20 mile range) from those things is going to run you 10 batteries, for 660lbs. And that's NOT including any other parts of the retrofit (like that 120lb motor, or the wiring [thick wire is heavy] or the electronics).