Looking at the pictures there is a rust stain that goes down from the plate into the top of the mast, and the plate itself looks like a mix of fiberglass and carbon.
The best way to fix it would be without adding metal, especially aluminum, which has worse corrosion problems worse and faster than steel, when mixed with carbon.
If you are willing to do all the work, you could repair the mast by gluing it back together with extra reinforcement, both internal and external.
It would be like repairing a broken femur for an orthopaedist, though. A serious job.
This is what I would try:
Drill/dig and carve out the cavity in the top of the mast to a good depth below the plate -hopefully 10-15cm, as far as practical and possible.
Use a high modulus or toughened epoxy that cures at room temperature.
Reinforce the inside of the top of the mast with carbon fiber cloth. 2-3 layers of +/-45 biaxial 200gsm.
Fit the broken pieces together and make sure they align correctly. Do what you must to make them fit correctly.
Drill holes in the top of the plate, maybe 10-12mm diameter. 4 or 5 of them, 1-2cm apart.
Buy premade solid carbon fiber rods to fit in the holes.
glue the old parts together with your epoxy.
make sure they are aligned correctly.
pour or inject epoxy filled with milled glass, carbon or microfibers through the holes to fill the cavity, minus the volume of the rods..
insert the carbon rods through the holes into the epoxy filled cavity.
let the epoxy cure minimum 24 h.
trim excess length of the rods above the plate.
sand the exterior of the repair from plate to 15cm below the break.
reinforce the exterior with minimum of 800 gsm EACH of unidirectional carbon, preferably 640MSI or greater modulus, AND 800 gsm +/-45 biaxial carbon cloth, preferably 640MSI+.
All surfaces must be clean and sanded to 60-100 grit.
Epoxy should be a room temp cure, "high modulus" and preferably "toughened" type.
Layers of cloth should all be applied ideally at once, or "wet on wet" or within 24 h of the preceding layer.
Ideally, the repair should be post cured at a higher temperature (typically 65-70 C) according to the resin supplier's specifications.
Do not try to use the mast for 7 to 10 days after cure, ESPECIALLY if not post cured.
If you want an easier job and don't mind a shorter mast, cut off the mast below the lowest point of the break.
Buy an ALL CARBON tuttle or KF to plate adapter.
Ketos and Horue can supply them, maybe F-one also.
Fit the mast into the cavity of the adapter.
Glue with epoxy filled with carbon or high strength fibers, as above.
Create a large fillet of filled epoxy around the mast, tapering down the mast 12-15 cm or so. This more or less recreates the original shape of the mast top.
Cover the plate and fillet, and an additional 15cm+ of mast below it, with unidirectional and biaxial carbon as above.
Doing either of these repairs will take time and effort and probably cost you $200+, and carbon can be a pain to work with, so you may want to consider buying a used or new mast instead, especially if you value time, the warranty, or lack tools and experience in composite repairs. But it's winter, sometimes a good time for projects...