shawnshank,
I think your GEBE setup can work for you as is. From my experience and from watching what others have tried, you need to be careful with your engine choice. The made in China Chung Yang 46cc or GP460 engine is too powerful and "peaky", doesn't have a smooth power band, and often poor reliability. I have one and it blew an engine seal on the first ride. Thr jerky power delivery causes belt issues and is hard on bike components as well. The clutches also engage at a crazy high RPM. I have a Goped Riot (sit down mini bike) that I have mounted a slightly modified Zenoah G43L engine on. It pulls well down low in the revs, has a great clutch, and is very smooth. I have the Riot geared for about 32 mph and it will climb any hill in my area with bogging. It could easily handle higher gearing. The G43L will hit about 12,000 rpm, and is built like a tank. Redmax used them in garden equipment, so you are looking at Tanaka-like reliability.
The G43L engine I have is from a Goped GSR40 TSI scooter. The cylinder for this model was mildly ported by Trevor Simpson. I added a fully adjustable carb, and otherwise it's stock. Since it hits 12,000 rpm, where my Tanaka 32 does about 7500 rpm, without a gearing change my speed would increase from 30mph to 45mph. I used a gear calculator and it says at 12,000 rpm it should be doing 48mph.
A stock G43L engine (no porting) should be a bit more powerful than a Tanaka 32 or Subaru 35 off the line and the extra revs would take you to the speed you want. The stock engine is rated at 2hp, but that's a conservative rating. These engines are easily modified, parts are available and you can get pipes and other HP stuff. These engines are not available new, so you have to pick one up off ebay, or off a Goped forum, which is where I got mine a few months ago. I paid $150 for the complete motor (in excellent shape), and purchased a new carb for $40 to get it running. These engine don't come with a gas tank. so you need to mount a Goped 1.5L (50 oz) tank to your lower mount strap below the engine. It's easy to do; I have done this and use it to carry more fuel for my Tanaka.