But part of the beauty of the R65 is its short wheel base - and you'd be giving that up just to mount a larger tire which will slow the steering, and the longer wheel base will slow it even more.
The smaller tire has less rolling resistance and allows one to corner quite aggressively.
It's a small, light weight, bike that is pretty damn good just as it is.
Take a look at the other bikes from that period - the norton commando, the ducati SS, the triumphs, BSA, Enfield, etc, ---- all small tires. It's just the way they were and they do just fine with them. They're not cruisers, or current sport bikes - it's just the way they are!