I can find plenty of outer repair kits and plenty of footwell repairs but this bulkhead is rusty at the top by the window bolt clamps inside!

Can't say I've ever seen them. I've got one to repair that's exactly the same so will have to make myself.

I have rot inside around the vent, YRM do outside full vent panels now. But they don't do the inside panels.

I was thinking I could use the outside vent panel but reversed/rotated as the angle is the same inside as the outside.

If you don't need the vent just bend some sheet in a vice.

This is what my corners look like