Once the beams are replaced/repaired, install a metal cap flashing to protect them.

I would get quotes and suggestions from a few different roofing professionals. You may want to continue the roof instead of keeping the beams.

This happened at a relative's house. The beams were replaced and the splice was inside the attic crawlspace. From the outside it looked as good as new. Protect the new beams so it does not happen again.

if those exposed beams go way back into the roof itself, all you can do is cut off the dry rot portion, or cut them all the way flush. This is frequent in my neighborhood since we have similar type of homes. we actually had to cut one off of ours because the beam went all the way into the structure. no way to replace it. you could try to cut and splice, prime and paint.

you must maintain these wood fascia boards and exposed beams. have yearly inspections from termite company, and make sure they are always waterproofed.