As you say, will need to strip and clean all the clutch parts and possibly replace the clutch plates?
Have you checked the gearbox oil?
Probably a good idea to change all the oils and clean the filters as it has been standing for a while.
There should be a seal on the footrest shaft which would have been cork originally (part 66-4902). The chaincase should only be filled to the level plug, my handbook says the amount should be 2 fluid ounces which is only about 60cc, if the case is overfilled then you are likely to get clutch problems.
The oil for the primary case needs to be a thinner grade than the engine oil, the handbook says SAE 20 oil, although using automatic transmission oil is a possible option.
You can probably get a photocopy manual from Bruce Main-Smith:
http://www.brucemain-smith.com/Looks like the manual you need is = 642/AVERY/1124 for '46-'53 models.