QS motors - who apparently are fairly well known for their hub motors will sell me a 3Kw motor for about $250 dollars. The choice to go with a hub motor is also obvious given the choice of bike: the vespa is not chain driven, the engine sits on the swingarm and provides direct power (via the gearbox) to the wheel. Going with a central motor would mean building a whole new drive system for the vespa. I have seen a post by someone who removed the engine and bolted an electric motor where it used to be, which would retain the gearbox and drive chain, but this seems unnecessarily complex. I think 3Kw is fine, given it's already more than the original 2-stroke put out. Though, it does not sound like a lot, given that my motorbike has 35Kw and is still underpowered. However, electric Kw are different from petrol Kw. The torque is always and instantly there, no waiting around or changing gear for that power. I am fairly sure that 3Kw is all I need as I am only aiming for a top speed of about 30mph. Helpfully Retrospective publish their motor’s stats as a 3Kw motor, and they claim it is of similar power output to that of a 125cc engine. So I feel fairly secure in my choice and keeping the motor power low will help me keep the battery low. The motor controller (I think I will use a Kelly Controller KLS7245N) actually costs more than the motor (somehow!), and postage is about $150. so about $700 for the whole kit (not including the batteries) from QS motors.