You could add a additional section of cylinder reducing the compression ratio and therefor less horsepower, you would be set then.
OK, just kidding!
Wondered if you do welding? There is a section of threads dedicated to chop....
Motorized Bicycle Welding, Fabrication and Painting - The Chop Shop Custom fabrication and projects, tanks, frames and more.
There are also some that put motorcycle engines on bicycles have have them look like the first Harley's. Others have interesting frames that are of themselves a one of a kind.
The drop loop or what ever they call it for fitting engine is a method for frame modification.
I've adjusted drop out slightly by cut and welding, but not the above. I wouold have but had only started welding when I was fitted the engine. Some put on a slant, but oil lubrication in crankcase must be considered if it can be done. I admit putting the engine on the platform I made for the engine has it contort to fit. Then the short spark plug has to have the screw top come off or the plug taken out and reinstalled after bolting down the engine.
Mud holds it together!