back in the day had quite a few, from 2.5hp on up. an old 3.5 techumseh in a spring frame became my 'mustang gt' of minis. I cranked it up to 5Krpm, used a larger carb, larger flowing exhaust, a larger cam, thinner head gasket and ground off the compression release on the exhaust valve (stem was like .005 too long to aid starting) chewed up case gaskets but 45+mph with an adult on it. 6 shoe centrifugal clutch, #35 chain and a rear wheel with a removable driven sprocket....
then one day I noticed the camshaft case support was cracking...fun while it lasted.