Get the R80, ride along for a few months and then decide if you still 'need' those extra HP. The torque and smoothness of the R80 engine will suit most so just give it a try.
(Then spent the money saved on the 1000cc conversion on suspension or brakes or anything.)
But to not-quite-answer your question:
The swap is simple, someone will point you to some thread. Swap barrels, re-jet the carbs and you're done. The heads are not a must to swap for they might work if you want torque. Most depends on your demands.