Depending on the vintage of the colt 5 you may only need a 7/8 shaft. I have seen colt's with chonda engines on them. 4hp engines were also fitted to the colt mini and I know they did not have a 1" shaft. The only trouble you may find, depending on the engine, is starting it. As it will not have a blade on the bottom, only the pulley, many briggs engines will be very hard to start.
GXV160 Honda engines are a fairly nice fit but expensive, they work well as they have a heavy flywheel.
