Hi Yodude, welcome to the madhouse. Sounds like you might be breaking new ground with that one? Of course, it's very likely been done by somebody, somewhere.
But you would need to find an SR tank, offer it up to the XS frame, decide whether it suits for size and the look you're after, then modify as required. As Buzzword says, you would need at the very least to change the front tank mounts or the frame. Probably less drastic to modify the tank, then at least the frame will still accept a standard tank . . .