most every airhead tank is interchangeable
WITH modifications to the rear mount.
The only other fuel tanks that will fit with no modification to the mounts are the G/S series. I don't know about the later GS series, but the earlier ones were the basis for the ST, so...
And with that knowledge, you might be able to fit a Paris Dakar tank.
The big drawback to those tanks (and others?) is that the R65 tank is longer than the others, so when another tank is used, there is a significant gap between the rear of the tank and the front of the seat.