These Lehr trimmers were mentioned here awhile back, and I think they are a great innovation for what they are.
The engine they use is a Subaru EH025, with what I understand is a Lehr developed propane delivery system. For a few minutes of trimming every week I think they are an excellent product. They run clean, start easily, and have no "spring tune up" carburetor issues from the cruddy car fuel that we now have to put up with.
One of the drawbacks for using one on a motorized bicycle is that they only use a 1 lb. propane cartridge, which has somewhere around the same amount of energy in it as a pint of auto fuel. Also, engines converted to propane have about 10% or so less power.
If I were going to try to build a propane bike engine setup, I would take the Lehr fuel system off of a trimmer, adapt it to a larger EH035 engine, and come up with some sort of refillable tank of around 3 to 4 lbs. Then maybe you'd be cookin' with gas!