When I got my '76 Standard 18 months ago (the infamous Lucille), the clutch was stuck solid as a rock. The clutch lever wouldn't pull in - the whole thing seemed to be simply seized. She had lots of other problems including carbs filled with stuff that looked like yoghurt and coffee grounds, a rusty fuel tank, badly corroded engine cases a toasted electrical system and no front brake functionality - but for some reason, the clutch was actually my major worry.
I looked at all sorts of remedies including putting diesel fuel in the crankcase and even disassembling the whole darned thing, but in the end, I simply lashed the clutch lever to the handlebar with a bungee cord, sat on the seat and rocked the bike back and forth (Fred Flintstone-style) on its wheels and after quite a while (maybe an hour or more), I started to feel a tiny bit of motion. After quite a bit more peddling around the garage, the clutch pretty much freed-up and once I got the engine running, the clutch has totally come back and it now functions perfectly.
Not that it is a particularly difficult or lengthy task, but I have never actually been inside Lucille's RH engine case to this day and now, more than 3600 miles (around 6000 km) later, all is well down there.