Only two suggestions I have is to find someone parting out a decent shape seat from around the same year that you could try and pick up. Other then that you could also try an upholsterer who repairs motorcycle seats. That’s what I did for my 300 KQ when I needed to get the seat recovered. They specialize in this field and should be able to carve you out a new foam and recover it. You’d have to call around and check, but that may be your best bet.