Welcome to the forum. I haven't built a motorized chopper bike but there are quite a few "Chopperologists" on this site.
Chopper builds usually require a little welding or buying a chopper motor mount plate from ebay. You will need to offset the motor to the motor chain side if you have a fat back tire.
Finally, most chopper/stretcher guys run a second chain tensioner or they buy or fab a spring loaded tensioner because the chain is so long.
This stuff is very routine for the chopper guys.