1 Locking washers were used in the past to stop nuts and bolts from coming undone, but the modern approach is to use a chemical solution: threadlocker. Do be aware of a few things though. There are many different types of different locking strengths for different jobs. Check the specification you need. The strength is greatly reduced by heat so do not use it on areas such as exhaust fixings — though heat is useful to release a threadlocked fastener which is really stuck.
2 So many things to check on a clutch! Apart from the plate thickness, make sure they are not warped using a truly flat surface (a piece of glass will do) and a feeler gauge. The slots in the clutch outer (‘basket’) and inner drum should be smooth and unworn or the clutch will never release properly. Check the release bearing and push rod straightness (roll it on the glass) and make sure all the ball bearings are where they should be.
3 Don’t re-use gaskets: they are meant to be for one-time use only. Some people use nothing on them, some use grease and some use gasket sealant — it is your choice, but if you do use sealant use it sparingly. Too much can find its way into an oilway, leading to all sorts of mechanical problems.