-Big custom fuckoff parts. They're not high tech but they're huge and they're a specific shape so you need a huge, precise and very expensive mould/die/whatever to make them. Anyone with the aircraft and a decent engineer could design a machine to make these parts but they would be left with a smoking crater where their wallet was after getting the mould made.

-Easy parts. Sure, an aircraft fuel pipe is worth 20k, but the civilian parts are made to higher standards anyways, we can find one no problem.

-Secret technical complex parts. Proprietary cutting edge stuff, which is frankly just bolted onto already complete aircraft. Obviously you can't replace it if you don't even know how it works, but the US doesn't let that stuff out of their direct control very often anyways.

Don't fucking talk to me about engines though, those are a whole different beast

TLDR: We can totally keep our F35s in the air as long as the parts we're replacing aren't the skin panels, the engines, or the Secret Third Thing. And as long as we have the money.