Well...
Assuming it’s broken off inside the fiberglas...
You may be able to drill and tap the end of the broken screw and back it out.
It is NOT easy drilling stainless steel. Get a really good bit.
You may be able to twist it out from the inside.
I don’t recall if you can see that position. A mirror and flashlight?
If you can see where the cleat is attached from in inside, you should see where the plate is imbedded in the fiberglas.
Point is, if the plate is large enough, you may consider positioning the cleat just off to one side, or above or below it’s original position.
Make sure you drill and tap the hole in the aluminum plate for the screw, then drill though just the fiberglas the size of the screw.
Hope that helps.