For the ST the NTC isn't avail as individual spare part, only the whole pump assy... big $$$ though...
But many NTC units used in other models, like Varadero, etc... will technically fit, same specs and that, one only needs to desolder and remove it from the stem and install it as replacement on your ST1100 fuel pump bracket.
There a number of threads in this forum from folks who'd obtained an NTC with identical specs at RadioShack or similar vendor, to fix the item in their bikes.
However, the Varadero part would have costed me about 70,- whilst I got the whole pump for 54,- and saved time... just dropped the pump in and that's it, took about 20 minutes in the garage.
And yes, also I was surprised over having to find rust there... yikes... seems the dew from the rear wheel spray accumulates there... (probably also the reason why reusing the gasket didn't work)
I cleaned and gently sanded the rim (it was only corroded at the last 2~3mm of the "lip") while placing a rag underneath to prevent debris contaminating the fuel and treated the metal a film of ACF-50, should do for another decade.