As I am new to B31 ownership some of my questions may appear basic stuff to you guys, I'm still on the learning curve.
Have owned Norton singles for years and had to deal with the notoriously leaky clutch cover. My 49 B31 uses lots of little screws and a cork gasket. Assuming both parts of the case are in good order is a new cork gasket assembled dry sufficient or do I need a sealant as well?
If its been overtightening in the past, there will be raised areas around the screw holes. You need to get the surfaces pretty near flat to stand a chance of being reasonably oiltight. The cork will give a little under pressure so a smear of grease both sides should be enough to make a reasonable seal. Light pressure with a screwdriver is all that you need, gradually tightening each screw in turn evenly. Don't overfill the chaincase with oil, just sufficient for the chain to dip into the oil.