Yes they do perform but these bikes weren’t built to do that when they were made. If you want to go fast buy a bullet bike.
One could say the same about a 1940 Willy's Coupe not being originally built to go Drag Racing or that a VW Bug was not built to be Baja Bug/ Dune Buggy/Street Racer/ Etc.
It's called customizing or in most cases, doing the best with what you have available that fits into your budget and build time frame.
Look at it this way, every time someone bastardizes an original it makes the untouched ones worth more.
Different Strokes, Different Folks.
One more thing, those Tecumseh's were not originally designed to be mounted on angle just to to try and look like the Japanese bikes that were pounding the death nail into the American mini bike market at the time.
That was a desperation rig made by the panicked underfunded mini bike makers.
Just saying.
Later,
Keith