Support for crawlspace beams

judging by the shape and cut of the wood, the method of assembly and the size of the members, i would assume you have a tile roof? either way, it looks like it was built for a very heavy roof load.

the rot you are seeing is just part of the problem, it most likely extends far into the timber and may even be present in the adjacent rafters.

bracing the beam (thats the transverse thing under the rafter) may not help much as it looks rotten as well.

the best thing to do is put in a post. this is a vertical column from ground to the bottom of the rafter. put down a wide block of wood or a concrete slab first to spread the load over the paving stones below. use a metal bracket at both ends and concrete anchors, lag bolts or nails to secure the post to the base and to the rafter. if you use a good solid post and are careful in your work, it should last until you can replace the roof.

To meet modern best practice you'd want both ends of any post anchored. Obviously your particular home isn't going to collapse if a post was to fall out (and it's unlikely it could anyway with that small of a gap).

To slightly derail the conversation, I'd seriously consider permanent adjustable floor jacks for your purposes. Why not build some future-proofing into your solution (assuming you aren't opting to dig proper footings)? They aren't terribly expensive (for the few you want to fix), and they'll allow you to add height down the road as needed.