If worse comes to worse you could always re drill the holes, tap them for threads, and use stainless steel machine screws to attach it back. It depends on the existing holes in the spar. Two years ago I replaced my Jib sheave block on my mast and I used machine screws.
They have these star rivets that you can use which only require a hammer to install them.
About four years ago when I bought my CDI Roller Furler, I had to install a Dwyer mast tang on my mast for my fore stay. I spotted the area where the tang was to be mounted, drilled the holes for the rivets and trailered my boat down to Rudy's store and had him rivet the tang on with his pneumatic rivet gun. It was well worth it to have him do it.
Last year I installed a pad eye on my mast for my whisker pole. I drilled the holes, tapped them for threads and used machine screws to attach it. I always use thread lock cement on the screws so they don't vibrate out.
It really depends on the condition of the holes on your boom or mast. If they're in good shape, you should be able to rivet it back. Myself, a rivet tool isn't something that I would wind up using more than once.