I think I have two options.
1: try and make a diagonal cut right through the joist, inside the loft, just before it emerges to the outside and make a new end which I can bolt/screw on.
2: try and build up the existing wood with filler/glue/small strips of wood. Let it all set and then shape it in situ.
There is a beam sitting on these but it doesn't support much, nothing in the middle actually, and the end joists, 2 at each end, are sound.
They are 180x100mm so quite chunky and getting in to do a diagonal cut will be tricky but I have made cuts like this before, in tight spaces with a small chainsaw.
There's a lot of wood to make up if I go the filler route, and the voids are obviously random in shape. Not easy to trim or machine up to make neat square edges to fit solid wood pieces back in.
The deepest the rot went in was maybe 150mm
These came out upside down for some reason!
This is one of the good ones: