You know I advocate car alternators - it's like "nothing beats cube's" -
If you do not want to use the crankshaft and have an adaptor made that goes through the side of the engine with axel-seals; then you have just about one single option left...
A guy who no longer are with us installed a car-alternator under the fender of the side-car wheel; he used standard pulleys - large on the wheel and small on the alternator => from about 50km/h and up he had just about full power from the alternator.
He who installed a Bosch 55A on my BMW R80 back in 1992:ish used B&S 2hp horizontal lawn-mover engines and car-alternators before he made his first crankshaft installation with pulleys on beemers back in the 1970:ties.
Since you have a side-car - this is also an option -
* crankshaft and pulley
* sidecar wheel and pulley
* lawn-mover engine and pulley