There are no F-35 parts only sourced in the US. Every country in the F-35 program has the ability to manufacture parts. The only thing the US controls is the firmware. The US can't lock us out of buying or building replacement components, they can only lock us out of getting newer firmware updates (eg, we can still use the firmware we have, just can't get anything past the point where they decide to cut us off).
Now that's a completely valid concern; being cut off from firmware updates would seriously degrade the capability of that part of our fleet over a long enough time horizon, which is one of the reasons why we're considering taking on a large order of Gripens as well. But it's not a "Stop your planes from flying" scenario. Even without the latest firmware we'd still be able to put an F-35 in the air and have it engage and destroy targets.