Now that I think about it, pretty much all of the FX bikes I've replaced headsets on from this era have been the cheaper 7.2 or 7.1. Not too surprised to see Trek changed up the headtube on the 7.3 FX.

If you grab a ZS44 headset lower that uses 45° bearings, you might be able to get away with reusing the old crown race, though it could be possible that things won't mesh up nicely. You could take just the fork in to get the crown race swapped if that ends up being the case.

If you want some extra durability, you could grab an EC44 headset lower intended to be used with a 1 1/8" straight steerer tube.