Been slowly moving forward with this project when I have free time and it's too cold outside to do anything else. Last fall, following the great advice in this thread, I cleaned the carb, flushed and freshened the oil and then hooked up a battery and a temporary milk-jug gas tank. The engine fired up after about 6 kicks, and it sounded and smelled just like I remember it did when I was a kid.
Since the frame was so mangled, I've decided it's "anything goes" on this project as long as I don't damage the wheels or engine, which seem to be the remaining valuable components. I spent a fair amount of time with the angle grinder, just cutting off all the modifications to the frame. Patient work with a hydraulic press and a dial indicator got the fork tubes back to straight, and I used a torch and a loooooong pipe to lever the downtube back into position.
I cut and re-welded the swingarm to bring the wheelbase back to stock configuration. I also re-did the shock mounts on the swingarm. Upper mounts will be tackled with the subframe, obviously. I just have something tacked in place for now.
Then the project sat on the shelf last summer during riding season.
This winter, I've been working on fabricating the rear engine mounts to mate the narrowcase frame to the widecase motor. In cutting off the the original mods, everything south of the swingarm pivot was gone, so I had to build that back up. I wanted to incorporate a center stand into the lower mount, which I think is going to work out nicely. I left the mounting plates quite bulky, figuring I could remove material but not add it back.
I think this is as far as I'll get this winter, as it will be warm soon and I need to get my two rideable machines ready for the season. Next winter will be front engine mount plates and I suppose a custom subframe, since the existing one just kinda dangles there now with the original mods hacked out.
I haven't welded since high school, and I was kinda hoping my technique would have improved more by now - lots of ugly welds, but I think they are at least sound. It's been a lot of fun so far.