I am almost finished with my chainsaw bmx bike. I have the saw mounted behind the seat on a "package rack". I am going to start with a v-belt driven system since I already have the pulleys and roller chain sprokets aren't alway cheap. Here is how I addressed some of the problems you may face.
1.) For the clutch of the saw I machined a collar that fits around the teeth on the clutch and then a 5/8" shaft fits inside the collar thaat can be removed since it is held by set screws. The collar will have to be welded to the sprocket. Since it has a shaft I can try different pulleys or gears until I get it right
2.) My bike has 22" high tires so I needed to slow the engine considerably. I made a jackshaft that sits under the seat of the bike. It uses a 1/2" live axle and rides on bearings. I can put different sided pulleys (or gears) on either end to adjust the gear ratio. I would have skipped this step but even with the 1/2" pulley on the engine and an 11" pulley on the other end I would have been going about 50mph at 3000rpm.
3) I didn't want a single speed bike so I ran all of the pulley so that at 3000 rpm (about where the clutch engages) my bike will go about 8 mile per hour but at 10,000 the bike will go much faster. But I also put a rear derailer on the bike so those numbers are for first gear. I will be abled to choose from 5 different gears on the rear wheel to adjust speed and cruising rpm of the engine.
This is all I can think of for now, if you have any questions please feel free to e-mail me.