A good engine rebuilder should be able to fix the head. The head looks like the damage was caused by spark knock, pinging or detonation(probably due to running lean). They will also be able to replace the valve seats and guides as needed.
As for the cylinder I would use a 77mm big bore kit since you already need it bored out. The price difference is small to none. You need to rejet already anyway from the looks of it.
I have an old 87 XR250R I'm building for next year. I found that parts swapping between my X600R with XR600R suspension and my XR650L is quite interesting.
Mine will have parts from both the 600R and 650L used in my build.
Here's a picture
Thats a XR650L seat and 4.7gal Clarke tank. It now has XR650L forks and rear shock. The spring rate is right on the money for my size.
I also found the rear rack from my XR650L will fit. The SuperTrapp exhaust is from my XL600R.
With the 4.7gal Clarke 650L gas tank leaves enough room for USD forks without any clearance issues.
I also used the clutch lever and perch. I'm using the choke lever as my decompression lever. It was simple to make work and it allowed me to use 29" wide bars.
Other parts will be swapped. The XR650L and my 87 XR250R have almost identical frames in many ways. The 650 frame also has reinforcements that are needed to stiffen the XR250R's frame. The seat fits like it was meant to, other then 1 bolt hole requires a new tab for it. It now has a 37.5" seat height.
The XR650L side panels fit perfect wit the XR650L's seat on the right side, the left side fits a XR600R side panel fits perfect also.
Here's a picture with the 250 side covers(cut for shock adjustment on the left side and a hand hold on the right side).
It makes carb access and rear shock adjustment easy.