Remember that these are nikasil plated aluminum cylinders. If you bore them out, you will need to either press in new liners or replate. Also, you don't want to hone these cylinders if they are plated already.
How do you know that the cylinder is toast? Lower compression doesn't mean bad rings. You could have burnt a valve. Have you added a bit of oil to the cylinder then rechecked the compression? If the comp comes up dramatically, the rings are bad. If the comp doesn't go up, you have a valve issue.
You can pull the head and cylinders with the engine inboard. Check the search function.
If your head is good, and you need cheap pistons, I'd consider picking up a used cylinder assembly and pistons off an 865 engine and use the 790 cams.
Also, if you are gonna bore the cylinders and are tight on cash,
THIS (CLICK) kit is relatively cheap. The old heads are interchangeable between engine sizes.
