Motor / Controller
Quite a lot to consider here :
Power / Performance / Type / Location ?
Ideally, I see little point in creating a 'slug' ! The converted scoot HAS to be as-good-as, if not better, performance-wise, than the original smelly scoot !
Generally, not always, but generally, with this type of 'low power' requirement, the motor and controller can be purchased as a paired 'kit' which makes this first step a little simpler!
Initially, I selected the required power (Kw). This can range from a couple of hundred watts to thousands of watts (Watts being the unit of measurement of power for an electric motor). An electric bicycle may have a motor power of 250watts right up to, maybe 1500watts / 1.5Kw (be careful of any legal issues here). To be fair, given that 'all' I want to achieve is to replace 'like with like' and that, typically, a 50cc petrol engine outputs a maximum 4 to 4.5horsepower (HP) then I need a 3 to 4Kw electric motor (1HP is approximately 3/4 of a Kw - a little less in fact but good enough for our purposes). It proved rather difficult to find a range of 4Kw motors but not so for 3Kw units.
So, selected electric power = 3Kw.
Type / Location
Realistically, two types of motor installation exist (assuimng that rear-wheel drive is to be maintained) :
 hub-motor - the motor is internal to (at the very centre of) the wheel and
 mid-mount motor - such a motor mounted away from the wheel and 'connected' to it by way of a belt, chain or shaft; similar in many ways to the original ICE motor.
Although a hub-motor has very distinct advantages (quiet and often the most efficient) it has the distinct disadvantage of replacing the rear wheel, which, in the case of low-end / internal rear-wheel drum-braked scoots (like this one) would need a re-worked swingarm with new brake assembly.
Being extremely keen to keep the scoot as standard as possible, ideally, I would NOT need to 're-invent' the swingarm or find a replacement for the rear-wheel internal drum brake ! This was mostly because I wanted to keep any final re-certification to an absolute minimum. As an additional benefit, replacing the swingarm and rear brake would undoubtedly be expensive and beyond the scope of (I felt) your average DIY-er ! No disrespect intended to you hard-core DIY-ers out there !
Changing the internal drum for a disc brake would usually, incur a change of cable actuator to hydraulic too... all additional and, as-it-turns-out, unnecessary complication and on-cost.
Remember, these are only my thoughts and reasonings... you people can do as you see fit for your particular purposes.
In the end I decided upon a mid-mount motor, of 3Kw nominal rating, via a chain drive to replace the original ICE motor and vari-drive / clutch / belt arrangement.
The challenge then was to find a way to mount the motor in the most efficient manner possible. Ideally I felt it should be mounted in lieu of the cylinder head / piston / crank shaft.
In the end I could not find enough space within the crank shaft / bottom end housing and mounted the motor horizontally on an adjustable bracket that simply bolted directly in lieu of the cylinder head. This effectively moved the drive shaft some 80mm forward of the original crank shaft BUT kept the motor on the swingarm meaning there was no change in chain length needed to be catered for from a suspended rear wheel. In fact the motor and rear wheel were always moving together as one unit on the original swingarm with the original rear wheel and internal drum brake.
I will blog the process (the good and the bad, the pluses and the minuses) and my reasoning as I go along . . .