A couple of things about the Duc single irk me, and it is far from perfect..;
The frame is going downhill, in itself not so bad except the bend at lower front of frame makes this more apparent... a big radius curve may have have helped that out..
The Tank is going downhill, that pretty rarely ever looks good. Also the front edge of the tank,angled to match front downrube, would have looked better, or maybe tank slid back.. at any rate those lines clash...
JMO.. FWIW..