If it's the subframe, I expect the seat and exhaust to also not be square to the wheel, is that the case?
Otherwise, check where the bolts (#46) mount the rear fender assy. to the exhaust, it should be cockeyed (not same on both sides).
If it is not square to exhaust, and #42 (bracket 6) in the drawing is bent, it can probably be straightened fairly easily. If not, I have one, but shipping cost might be prohibitive.
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