Predator 212 question

I copied this from another recent thread of mine:

I have a 6.5 greyhound that doesn't have charging coils. I bought a 2.5" CIM motor and I plan to run it as a generator off a sprocket behind the CVT on my gokart. The sprocket would take the place of the spacer between the engine and the fixed drive face. I just need to design a regulator for it. If the motor has enough torque to turn over the engine it'll double as a starter. I plan to undertake this project in the winter.
This seems to come up quite a bit. I'm not a young man. In my day, there were bicycle generators that could operate a headlight.

Why, oh why, in this day and age of LED lights, and lithium batteries.. Is this so tough?