Welcome aboard, Huw. Nice looking modified LS you have.
At first blush, regarding the engine not wanting to rev beyond 4K, I'd be looking at the CV carb's diaphragms. The slightest pin hole or tear in the thin rubber will often reveal itself with the poor performance you've noted. Always use OEM Bing diaphragms. Cheaper repops are available but they fail to provide quality performance.
If only one enrichener circuit is funtional, there's no wonder the engine doesn't want to play nice until it's fully warmed. At least you already know what the fix is.
"there appears to be a leak in a gasket or valve as air escapes out of the top between the left cylinder and head making a slight hissing sound. there is also a very fine deposit of oil around this area." Could be a failing head gasket. The oil would be another clue. Perhaps check valve clearances and cautiously re-torque the cylinder studs; pulled studs are not uncommon and are a PITA to repair.
Riding farm trails and fields on an R65LS really won't provide a fair test of the bikes handling. Remember, you have an LS rather than a GS.

The lower shaped portion of your fairing looks just like the Maier that's on my '81 R65 but with a single headlight.