There was an XLS Ironhead model that used a Suoerglide's fuel tank, (oh, I remember, it was called an XLS Roadster) and this seems to excuse the general habit of fitting the larger bob tanks on Sportsters, as far as ICBC is concerned. The rules for mc Collector status allow any period luggage and/or fairing and/or seating, so, given the changes a fairing makes to the looks, fittings, and handling of a motorcycle, the resulting 'stockness' gets pretty debatable. Hans could put a 30 lb Windjammer on his Yamaha 125, reinforce the frame and beef of the front suspension to carry it, add big brakes to slow the additional weight, change the handlebars and mirrors,and delete the stock headlight and turn signals and associated wiring - but he cannot have a tiny dent in the tank.