Hey, I have an '81 H16 and I was planning to redo all the hardware on the mast this past winter. I looked at all the rivets carefully and they seemed to be in good condition so all I did was drill out the rivets for the top and bottom castings so I could remove them and take a look inside.
Among other things, the H16 mast needs to float and so it needs to be watertight. When you replace the foam plugs and the aluminum plates make sure the aluminum plates are sealed in the mast. Then make sure that all the rivets between those two plates are watertight.
If you redo any rivets between the plates you will need a anti-electrolysis compound (Duralac is an excellent choice I hear) and some "rivet caps" (one "rivet cap" for each rivet). To replace the retainer, first put some anti-electrolysis compound on the outside of the rivet caps, put the rivet caps in the holes on the mast for the retainer, line up the retainer over the rivet caps and rivet it onto the mast.
A few points:
In general, corrosion on aluminum forms a barrier against more corrosion, so unless you are going to paint over where you cleaned off the corrosion, don't clean off all the corrosion!!
Use a sealant for the aluminum plates inside the mast, but use an anti-electrolysis compound for rivet caps.
Don't use any glue on the top and bottom castings when you put them back on the mast. Two rivets on the top and what is it 5? on the bottom is plenty.
Don't use aluminum rivets. Use stainless steel or monel rivets. Monel is stronger, you will get less corrosion with monel, but you will need a bigger, better, stronger, rivet gun to do monel rivets. You will (unless, of course, you are Chuck Norris)