Can resistive fuel senders/transducers be repaired/reconditioned?
There are shops that do this and egid gave you a couple of hints on how to find one, but your local mechanic can probably recondition your fuel sender about as well as any of the specialty shops, as long as they're not squeamish about opening up the unit (and as long as the unit can be opened up - I'm not sure about the TB20 senders).

What's basically involved is cleaning the resistor element and wiper contacts, making sure everything is making good contact through its full range of travel, and verifying that the sender is still within a reasonable calibration range when you're done with the cleaning (you can use your other, working sender to determine what's "reasonable", or verify using your fuel gauge).

The Comanche folks have a nice article on reconditioning a (really beat up) fuel sender, and technologically they're similar to automotive fuel senders, so this article about reconditioning Corvette fuel senders may also be useful to get an idea of what you're in for.