I'd say, it's actual monotery value is pretty limited. Fact is, not a signifigant number of X1 nor S3Ts(S3 and S3T use the same tank) were built for there to be a large calling for remanufactored fuel tanks. On top of that, a large majority of tube frame Buells have been mutilated, destroyed, and chopped up into piece of **** choppers.
From a strictly Buell lover/collector point of view, it is a very unique piece and would be loved/cherished. Value as such is simply what someone is willing to spend on it. As far legality of it, the piece would indeed be legally owned. As long as one was producing tanks as replacement tanks/reproduction tanks, and not "original Buell" tanks, I believe you'd be fine. Lots of companies make replacement parts for vehicles of all types that are in no associated with the OEM.