Yeah, I'm still here.
The project is literally not off the table yet, sitting on one of the benches in my garage. Recently got a set of 690 Rally tanks for trial fitting, those would work a lot better so I sold the 4x 660 Rally tanks.
Right now one option I'm considering is to have a pro put the stem and rear part of the frame in a jig, and then make up some new trellis frame tubes to connect them. Basically make it look like a 690 frame except a bit wider in the middle. That might actually work...
The F650 engine is the second option I am considering, especially if I can manage to find a crashed Husky Terra. I do know that it's a big wide engine but I've never sized it up to see if it would fit in the frame. Since it's got a single throttle body I think it should fit in between the frame spines on top. Worst case you might have to redo the two bottom frame tubes, but that's not too hard.