I'm not sure, but I'm pretty sure they're metric.
If you still have enough of the head, just cut a slot in it either with a fine hacksaw blade or a dremel tool, use plenty of anti-seize, and reuse it.
Another alternative is to find the closest slightly larger US size and retap. Use coarse threads rather than fine, and it can be very close in size without problems. It does not take all that much thread to hold the TPS in, and it need not survive an atomic blast.