rack mount
I have a Ryobi 4 cycle, 26.2cc (rack mount). The only mechanical problem is the starter cord won't retract. It's from an unfinished project and has maybe 1 or 2 miles of use. Included are a good plywood mounting box, an aluminum rubber covered drive shaft and a pillow block shaft support bearing. The Ryobi has a centrifugal clutch.
What it needs...A longer throttle cable and mounting hardware to attach it to your bike. I'd also recommend a different shaft support bearing as the one it's got is overkill and too stiff. A 1 inch car wheel bearing would work better.
Sorry, no picture but you can see the engine at any Home Depot. It's from a weed whacker (never used). If interested, make me an offer. I'm in Louisiana.