You need some frame bracing, especially since back end is gone. The pressed up top tube (
) flexes a bit with twin shock rear section just below tank flexes a lot and tube is almost always distorted enough for paint to flake off. Friend did a 500f 'mono 'probably 1083~85. Ended up cutting complete top section out, welding in a bunch of tractor exhaust tubing (something DOM, he worked at a farm place) Ended up looking like a 'real 'frame. Don't remember what happened to it, he got into heavily modified big bore GS1000's on the street (180+mph 1425cc, etc)