The mast base is much easier to fix than the step. You'll need a rivet gun large enough to pull 3/16" Stainless Steel rivets. I bought mine at a auto supply house for around $40.00. If you don't have one and your boat is on a trailer, you could probably bring it to a marina (or maybe a body shop) and sweet talk them into putting them in for you. It would only take them a couple minutes. You can pick up Stainless Steel rivets at any sailboat dealer. I think you will need the 1/2" long variety.
Good luck,
Jack