Still haven't picked up a wrench on this project yet, but I have been thinking. Here is where I am at...
Having decided I want to do this with buying as little as possible, and I am abandoning the belt drive plan. I have a stack of old bikes around, but nothing nice, so I want to be able to move this between frames easily when I do find something. Also, I have decided I want to add a homemade jackshaft to it, so the plan is a piece of 90% stock coming up from the platform for the engine with a bracket on top for a rear wheel hub to fit in behind the seat post tube. The engine has an 11 tooth #35 sprocket on the clutch, and I have the 70 tooth sprocket on the rear wheel of the minibike I can scavenge. The idea is mounting it to the left side of a rear hub turned jackshaft, and a gear on the right side will drive a chain to an internally geared rear wheel. Another chain from the pedals to the freewheel on the jackshaft completes the drivetrain.
I don't know the ratios inside the geared hub, but I know second gear is 1:1, and that is enough for me to work with. Shifting into top gear when I hit 40 km/h sounds about right, so assuming an 8000 rpm "redline" (*see footnote) I still need around a 4:1 gear reduction between the jackshaft and the rear wheel. Does this sound right to those of you who do this math regularly?
Footnote: I have no information on this engine in particular, I picked the number 8000 based on what I have read here about other 4 strokes. The circumference of a 26" bicycle wheel is double that of the rear wheel of the minibike, and the minibike goes about 46 km/h as it is currently configured, so that approach to estimating says I need a 2:1 ratio between jackshaft and wheel...