My truck is different than putting a cvt on a bike, as I run the cvt belt from the engine directly to a cross shaft to drive both wheels. I also have electric start, and I raised my motor up 1 inch. If I did a bike with a gx motor, I would run a cvt to a jackshaft under the cylinder. This would clear the exhaust, and you would not be limited to the plate mount they give you. There are longer cvt belts available to put the jackshaft where you need it, not where the kits think you need it.
IMO, a Honda GC engine would be a much better choice for a simplex, as the exhaust is in front, the carb is in the back, just like a simplex motor. Also, the cylinder is upright, and looks much better. The only downside to a GC is the lack of charging stator for lights. This could be overcome by running a small generator off the jackshaft to power lights.
Rick