I haven't built a weed whacker bike but I thought the thread needs at least one sensible reply lol.
My first motorised vehicle was a 27cc mini scooter, and with my weight which is similar to yours it was able to accelerate reasonably and climb reasonable inclines with the gearing it had, but that gearing obviously limited top speed, around 20-25mph, above that the little engine was revving its nuts off, which quickly killed it.
Having the ability to pedal along with the engine power you should be able to get a better top speed with still reasonable acceleration and climbing, as long as the friction drum has sufficient traction on the tyre.