Acorn nuts are used on both rivets and bolts.  Scamp uses very few bolts in place of rivets but they do use them occasion.  I found at least one on my new Scamp and the bolt was just long enough that the acorn nut started to unscrew.  I have since put on a nylock nut (not too tight) and the acorn nut on top of that just for appearance.  I hate to send you to a different forum but in this thread, Post 34 details my discovery of the bolt:

http://www.fiberglassrv.com/forums/f54/yet-another-rivet-question-76039.html

All the answers to your questions are on the net so please do some searching.

Here is Scamp HOWTO for replacing rivets:

https://www.facebook.com/297691030279431/videos/vb.297691030279431/113727918754768/?type=2&theater

Many people add a very little butyl to the rivet (or bolt), and a dab of sealant on the head of the rivet before putting the snap cap on.

As for rivet size, this will get your started:

http://www.fiberglassrv.com/forums/f55/scamp-rivet-size-list-52225.html

You might have to use slightly longer or shorter rivets then what was originally used to account for changes with age, compression of the rat fur, etc.