I replaced my heat pump about 1-2 yrs ago with a Trane XR13 unit. I now need a new air handler installed in my attic. What Trane air handler unit would be compatible with the heat pump?

Any, as long as you get the same tonnage as your outdoor unit. Also I would get a expansion valve for a freon metering device instead of a piston.

True any brand will work and is compatible with your outside system. You can go 1/2 ton larger on the indoor unit(max) to bring your SEER up a little bit.