Most any 4 stroke engine of the HS type mount to a flat plate with 4 holes.
I if you want a big 4-stroke go with a motorcycle engine, just use a frame suited to the purpose, like a MotoPed.
A full suspension dirt bike frame... with pedals.
It takes a Honda H2 engine mount.
That engine was a 4-stroke PCC Motors 147FMF 88cc.
$2K+ for the bike, ~$120 for the 3 speed engine.
It would fly, that's for sure.
I find the tamer more legally accepted ~50cc range engines to be more than enough power on bicycle when you use it's own gears in the power train.
With an internal 3-speed rear hub it jumps off the line in 1st from a dead stop well, climbs hills well, and tops out around 40mph in 3rd.
I have built most every type of MB over the years.
Dollar for dollar this even beats out electrics for just flat ease of use, reliability, ease of maintenance, distance, and just plain old fun, mostly because of the cost and distance issues with electrics is all.
Here is a shot of the build I am working on now while it is easy to see the drive train.
It has the larger HS 144F-1G 53cc engine at the start of the drive train, my 10G KCK long shaft conversion of the Skyhawk 4G model B 5:1 reduction 20 tooth - 100 tooth freewheel bearing pulley transfer case, a 2 sprocket freewheel SBP bottom bracket setup, and a Sturmey-Archer RX-RK5 5-speed internal shift disc brake hub at the end of the train.
Gears change everything.
I just devised a better power transfer system is all.
2 easily adjustable chains and a belt.
One you can still easily pedal even with the engine off ;-}
Anyway, I ramble...
Just sharing my vision and projects of what I find ideal for the masses transportation needs.
If you like the concept let your Do It Your Self juices flow like I did as I couldn't be more pleased with this creation ;-}