Welcome to the forum. Two stroke frame motor will fit with welding or if you buy a chopper motor mount from one of the motor bike kit vendors on ebay (like boygofast). One or our sponsers probably sells them as well. I think you need an electric motor to be street legal in the UK.
Check out our photo gallery to see some motorized stingray choppers. It it much easier to use a steel hard tail mountain bike or beach cruiser for a first motorized build. Chopper frames and fat rear tires create additional problems to be solved. You will see how our members handled those problems in the photo gallery.
“You're only young once, but you can be immature as long as you like" -Unknown