First, I cut out the crotch zipper and buttons in a small wedge, and tried them on to figure out the additional waist inches I would need. Then, I used the strips I had cut out of the legs (you can actually see the inseams in the photos), and fashioned them into a replacement crotch. I guess from the picture you can see the basic pattern involved, with new wedges of fabric stitched into place and then new buttons riveted on, with new buttonholes created on the opposite side. The overall effect is a little weird, and very different from how normal jeans look, but I don't mind that. It's kinda fun.
There were big tears in the legs, and I used iron on mending tape to patch them up, and then stitched round them to negate the tape's propensity to peel off after a while. For my traditional crosses on the back pocket I tried a hand stitch rather than dye pens. The effect is rather rough but I think that is fine given how the rest of them look.
Next up: Not jeans!