If you are happy with the wheelbase and you don't want to stretch it out anymore, then I would work off of what he has done already.
IMO: I would bend up some 1" DOM tubing and weld in the lower frame rails (green) while the swingarm is still welded to the dropouts. Then, I would cut the swingarm off the dropouts and unbolt it from the swingarm pivot. After you get rid of the swingarm, I would trim back where it was mounted as far as possible depending on where your rear motor mounts are. You may have to fab a triangular gusset where the new lower framerails come in if you can't cut back the original brackets all the way down to the tube.
If you can post more pictures of the rear motor mounts I may be able to make a few better suggestions for that area as well. Let me know if I can help anymore!