There are several attachment options. If you go with rivets, you may want to consider using "aircraft" grade rivets, so that the inside part is sealed off.
You can also get thread repair inserts that have a large male thread on the outside diameter & a smaller female thread inside. These are sort of a beefed up version of a "heli-coil". They come in many sizes & materials.
You can also get swaged-in "Pem" style inserts. These are basically a giant rivet with a female thread on the inside. This is the repair that I would probably choose.
The strongest possible repair is probably to have a guy with a TIG welder put a tapped stud in there for you, but on a mast
that is on a boat that is in the water
, that isn't going to be cheap
nor easy, even if you can find the right guy that knows how to do that properly.
If you decide to go with a backing plate, & you need to bend it, & you don't have access to a bending machine, you might try c-clamping a flat plate to a large pipe & hitting it with a large dead blow mallet. It's a little labor intensive, & you need to start with a piece of stock about twice as long as you need to end up with, but I've had good luck with this method up to about 3/16" thick flat plate. Try to use a pipe that is about 20-30% smaller in diameter than the radius you want to end up with.
You could drill large holes in the opposite side of the mast
that are big enough to clear the head
of an Allen screw & put an Allen nut in from the back side, but I would try pretty hard to avoid abusing a mast that way if at all possible.