Just at a quick glance I would say if you want to save some money, you probably did not pay much for the boat, use the same base. Chip off of the base anything that may flake off. Make sure the walls of the mast are no thinner at the 1/2 inches on the bottom as they are above that. A similar thing happened to my boat a bit over 20 years ago when I was sailing H16s. You would like to avoid further electrolysis. You can use a dielectric grease, a teflon compound, or ignore it and just place some silicone sealant on it. There is a foam plug a bit in from the base. While the bottom is off put some extra sealant on that to make sure you mast stays as watertight as you can get it.

Drill in different spots and you should be ready to go. The only time the rivets see any stress is in raising and lowering the mast. While the mast is up it is a compressive load and does not effect the rivets.

Have fun sailing,
Dan