So I had this idea when I was a kid of building a motorcycle using a small automobile engine, and even though I am older and should be wiser, I am not. I still want to build one. Originally I wanted to use a 3.5 Liter aluminum V-8 engine, originally developed by Buick and later sold to Triumph and Rover. The same engine was used in the TR-8 and in the first V-8 powered Rovers (Land & Sedans). 3.5 Liters may sound like a big enough engine for a car now, but at the time (mid 1960's) it was minuscule. Muscle cars were using 427 cubic inch behemoths. 427 cubic inches translates to 7 Liters, twice the size of the little V-8 I had fantasies of using.
Nothing ever came of my idea, but several other people have been bitten by the same bug and gone off and actually built them. There is even a company building and selling them. They are monstrous brutes and I imagine they would be a thrill to ride. They have got to be heavy, though, and that means you have to be careful with them, especially at slow speeds.
The main motivation for using a car engine in a motorcycle is cost. Car engines are mass produced in far greater quantities than large motorcycles. You can get a great deal more power for a lot less money. Another advantage is reliability.
Factory produced motorcycles are built like airplanes. Everything is light weight, specially designed for this one purpose, limited quantities, and very expensive. Highly stressed machines like this require fastidious maintenance. One little error and a five thousand dollar engine goes up in smoke. On the plus side you get a high performance machine that will fly through the curves, compete with jet aircraft for acceleration, and stop on a dime.
But what if you just want a cheap bike that will boot, scoot & boogy? I'm thinking you might be able to get away with a motor and tranny from a Geo-Metro. I mean 70hp out to be enough for a bike, eh?