The real-deal ST670 is over $200. I needed a replacement, did some research and stumbled onto one of these: https://www.ia.omron.com/product/item/1377/.
The barrel of the E2E-X2D1-N 2M sensor is thicker and it is only about an inch long, but electronically it is a close enough match that it works. I suspect there are other OMRON sensors that may be a closer match as far as dimensions, but this one was cheap and available almost at once, so I quit looking.
There is a very simple spec sheet online for the ST670: http://www.autometer.com/media/manual/2650-1749-77.pdf. I used it to determine the wiring polarity.
These sensors, or perhaps more likely Chinese knockoffs, are available on Ebay for as little as $10.