650 Big Bore Kit

I doubt you could go much bigger with a standard 650 barrel as the bore-headbolt distance will get very narrow, and the barrel skirts that go into the crankcase will get dangerously thin. If you went for liners, then the crankcases will then need machining too.

Looking at the differences between 650 and Atlas should tell you all you need to know.

Then becomes the 'to achieve what?' question. The Atlas, with an extra 100cc is barely any different in performance to a 650 and vibrates more. That said, it does have more mid range torque. 

Just my thoughts, others may well say differently. 

Regards, George. 

Back in the late 60s & early 70s Dunstall sold a big bore 810cc kit aimed at the  Atlas and Commando 750 motors. For you cash you got an alloy barrel and a set of high quality pistons. He later offered a version of this 810cc kit to 650 owners. However to get it to fit required the crankcase mouth to be opened a little and the cylinder base flange studs replaced by a set of  stepped 3/8" / 5/16" items. 

The downside of this conversion was the need to rebalance the crankshaft to match the 810cc pistons. Failure to do so generally led to vibration worse than most  standard Atlas bikes.