Yeah, you can try advancing the timing to see if it helps. It can either be done with an offset key, which you can either make or you might find at some of the suppliers for the little scooters and "Go Peds". You can also just clean the crank and flywheel real well, and put it on without the key; just using some loctite. It won't move. A larger bore carb than stock, and a free flowing muffler with no baffles and two large outlets will help as well. You will probably find it will like more like a 14 x 8 prop, and the APC props are efficient and good to use on small motors.

This size motor doesn't usually make a good conversion because of it's poor power to weight ratio, but hey, tinkering is fun anyway right?

Have fun,
AV8TOR