This practice was very common in the early 1970s, done by converting the rear wheel to a 16" rim (I laced a bunch of them myself) followed by various 5.00x16 or larger tires being mounted. The limitation was the tire width rubbing against the brake arm on the right side and the chain guard on the left. The brake side isn't hard to modify: just remount it off to the right side of the U-hanger bracket, and if that's not enough it can be bent in a gentle radius away from the tire. For a while there were custom offset rear brake arm mounts, usually in Chopper magazine (or others with similar bents), even chromed ones.
The limitation is really the chain guard. The custom chopper chain guards (a 1" wide chromed strip bent in the profile of the top of the OEM guard, with mounts attached) were commonly used, but aren't much of a 'guard'. On the K0/1 bikes this let a LOT of oil find its way up to make racing stripes down one's left shoulder.
In those days there were not many round-profile 5.00 or 5.10 width tires, though. Today there are, thanks to the grippy needs of modern superbikes (of which this bike WAS the first...
), so having a rounder profile rear would go a LONG way toward improving the handling and stopping the 'head shake' that is caused by squared rear tires. I'd sure suggest a steering damper if you go that route?
