According to info available, the Honda GX50 is 10.8" long, 7.7" wide(pullstarter to bellhousing) and 13.9" tall. It weighs 12.1 lbs.
My Mitsubishi TLE43cc engine is 9.625" long, 6.75" wide and 10.25" tall. According to my bathroom scale, it weighs 9.4 lbs.
My GP460 engine is 7.25" long from carb to exhaust port, 7.25" wide and 8.25" tall. It weighs less than 7 lbs(estimated).
My pocketbike engine is 9" long, 6.75" WIDE and 4.75" TALL!!! It weighs 7.8 lbs.
The 460 and pocketbike engines are even smaller in mass/dimensions than Honda/Mits engines. This is because H/M engines are boxy; pocketbike/460 engines lack massive plastic carb and exhaust covers.
Pocketbike engine is VERY short. You could stack TWO Cag engines in the space of a Mits engine, and almost THREE pocketbike engines in the space of a Honda GX50 engine!!!
So the misconception of wide pocketbike engines are grossly untrue!
The 3:1 pocketbike transmission which widens the engine is not needed, when used in conventional engine drives. If frame-mounting the pocketbike engine with this 3:1 gearbox, a 9:1 jackshaft will be needed to transport power to the cassette/shift kit.
The 5:1 pocketbike gearbox can be utilized, placing the engine sprocket in correct position, without using jackshafts. A 28.6:1 gear ratio is easily attained by using 11-tooth gearbox sprocket and 63t drivering sprocket.
JMO, mounting a pocketbike engine with a Scooterguy mount and a mix of pocketbike sprockets and SBP components would be a compact and powerful combination.
I might do that.