If you can find a flat surface, a piece of glass, a metal plate, even a piece of thick plywood or board that is straight would work, I have a small granite surface plate that will span the two tubes and fit between lower triple and fender, it's easy as can be to do.
The XS1 has external springs and gaiters so I'm checking between the triples, but below the lower works the same, since you know your tubes are straight.
The gap is visible here and the plate will "rock".
When I disassemble I will straighten the lower. It's pretty easy, probably could be done in place with the wheel and fender off, and tubes lowered one at time. With clamp bolts "just" loose, use the tube as a lever. Adjust (bend) till both tubes align with and easily slide up into the upper, done.
Since we are talking about the cast, or forged? lower triple,
An interesting discussion of cast iron vs. cast steel