G'day All, I did buy a quantity of solid aluminium rivets round heads 3/16" dia from a company in Sydney some time ago but it went belly up, I still have a supply of various rivets and I have bought larger diameter rivets from the USA, found them in a google search, and as for setting tools I modified my air chisel tools to do the hammer bit and I made the set tool on my lathe but that don't help you.
One thing I can tell you though is you will need 2 other people to help you re-rivet the bonnet anyway good luck with it cheers Dennis
PS Yes RobH I tend to agree that pop rivets look out of place where the round heads should be especially those around fuel fillers etc...