I don't have a tire roller type engine, but I did deal personally with Jim (I believe thats his name) of livefastmotors.. He was a pleasure to deal with and let me pick up my motor in person. Real nice guy.
I have a livefastmotors unit I have been very happy with. It is the rear tire roller model - just got it this winter, and have had it on the road maybe 100 miles or so.
Heck the screws are tight on the motor. That is good. And the drive unit is heavy duty. Had one problem, see my thread in "rack mount" section, with the clutch disk spinning off the roller shaft going down hills (and locking up the roller) which I solved by putting epoxy on the roller threads before threading the clutch disk back on. Doesn't happen anymore.
Has pretty good power and will go up some reasonable hills, but not as well as my center mount 70cc (i have one of these too). You have to be sure that the drive unit is clamped firmly down on the rear wheel, and check it from time to time to be sure that it hasn't vibrated loose.
The engine is noisier than my centermount, due to the short exhaust pipe I think. And I have noticed that the engine appears to go thru some periods of running roughly. I am running 24:1 in it. It usually straightens out if I back off on the throttle a little OR accelarate - I play with the throttle. Don't want to change my gasil mix as I am using the same batch for both my center mount and this tire roller unit.
It really shines on the flat, zips along pretty good. I feel it is more reliable than my center mount, although I think I have finally solved most of the problems I have been having with my center mount. Less parts to fail, no chain, sprockets or clutch lever etc. And so what if the tire wears out after 1000 miles, a new tire is 10 bucks - big deal.
Solidly built unit, but I think people need to be aware about the clutch problem I had. If you think about the design, as long as the engine is applying pressure to the clutch disk, it is being wound ON to the threads. But when you are going down the hill, and the tire is driving the tire roller faster than the engine (the engine appears to cause some drag on the clutch disk even at low throttle) then the clutch disk can UNWIND from the threads, clamping itself up tight on the engine and siezing up the tire roller !
I think there needs to be a cotter key or something in a hole going through the clutch disk and threaded roller shaft to "lock" it to prevent this from occurring.
On first start, you need to raise the engine off the wheel, when you can idle it - then you clamp the motor down on the wheel. I have a lot of hills around here - in NH, and it does remarkably well. The few hills I have to peddle on, the peddling is easy as you are just assisting the motor.
Be sure to tighten the frame mounting bolts from time to time, as the unit may shift a little so that the roller is "crooked" on the tire, which can wear the tire, and reduce power.
Pretty easy to install, although the instructions weren't very good
You know, for the same price as his 4-stroke model, you can get a Robins-Suburu from Staton... And, I speak from experience when I say that Staton's friction drive kits are as good as they get. Heavy duty, roller supported at both ends, no possible spin-off problem. I haven't had a single problem with either of the two I bought from him. (one Honda, one Mitsubishi)