about that...my whole frame is 35mm & i thought i was gonna have problems. but, even tho the mounts don't seat all the way, the studs (very tightly) clear the tubing and the brackets fit on enough for the nuts. the brackets formed themselves to the bigger tubing & the engine-mounting is rock-solid.
I used a small grinding wheel with my zip-saw (heavy duty Dremel) to widen the curve in the mounting blocks for a huge framed Jamis cruiser. (Wider, not any deeper) Fit better than OJ's gloves. I didn't get any pics from this build & the bike went to Florida. This bike was tall like Augi's.....I could reach the pedals but not the ground.
One way you can mount to different sized tubes is to make a steel plate drilled to fit the motor mount studs and another set of holes to fit a vibration damping u-bolt that fits the tube. I've done this on two bikes without any problems.
Here's a pic of an 80cc mounted that way.
Here's a couple of different kinds of cushioned u-bolts available in different sizes at McMaster-Carr