The flange bolts and flange nuts crenwelge linked to are what I use for bolted frame pieces.
For riveted frames I re-rivet them. It's not real easy but can be done and done well even for us "hobby" folks. Ideally we'd do them cold with a huge/strong hydraulic press but that's unlikely to happen these days. The way you can do it at home is hot and set them with an air hammer or better yet a real rivet hammer and the appropriate dies. Heat the rivet red hot then install it in the frame and set it. Hard to get them tight if you heat them already in the frame.
Here's the sort of rivet hammer
http://store.e-airtool.com/index.php?main_page=index&cPath=92_163Rivets can be bought at these places.
http://www.bylerrivet.comhttp://www.mcmaster.com/#standard-rivets/=lj6bddhttp://www.bigflatsrivet.com/Grigg
