One reason I have not done a DIY (non-kit) build is because the 2 stroke weed eater and goped engines are so loud. I may be willing to do a bike with a 4 cycle weed eater. I avoid the Briggs and most HFs because I want to maintain a street legal displacement for motorized bikes.
I think someone needs to buy one of these engines and develop it for motorized bikes.
It's a great value if it can be used and it's 4 stroke. There aren't many 4 stroke alternatives to RS and Honda, so it's worthwhile to take a look at this engine.
I doubt that the B&S 34cc engines would be good candidates for motorized bicycles. They are a side valve (i.e. "flathead") engine, which typically produce somewhere in the range of 60% to 70% of the power of a similar size overhead valve engine. Given that a 34cc overhead valve engine doesn't have excess power for an MB to begin with, I think that a flat head 34cc engine would be quite a disappointment power-wise. B&S also makes (or made) a 40cc version of this engine, but that's still only a little bit bigger than a 34cc engine.
Additionally, I think these engines have an overhung crankshaft. That is a good point for a model airplane conversion, but the extra width isn't so good for an MB engine.
For this class of engine size, I think the Honda and Subaru are still the engines of choice.