I've seen replacement thermometers in the past on eBay. And they were the correct ones for the round dial moto-meters. Not the replacement thermometers for the newly made thermometer equipted Flying Quail hood ornaments. My success rate for restoring original Moto-Meters ran about 50%. That is freeing up the thermometer fluid so the meter would actually work. I used boiling water, a bowl of ice and a length of strong twine.
Disassemble* the Moto-Meter, place the stem of the thermometer in the boiling water for a few minutes. Then place the stem in the bowl of ice for a few minutes again. Now tie one end of a 3 or 4 foot length of twine securely to the top of the Moto-Meter dial and start swinging the meter body around and around for a little bit. The centrifugal force this creates helps in getting the fluid to move. Make sure you have plenty of room around you first! Repeat this procedure until you either free-up the alcohol in the thermometer or realize it ain't going to work. Not to many years ago decent "original" Moto-Meters were not to hard to find and fairly priced. EBay use to be a good hunting ground, but I got tired of people trying to sale older repop Moto-Meters as good originals and the ridiculous money they were asking for them.
Bob-A
* This does not include removing the glass thermometer tube from the Moto-Meter body. The tube is secured to the body with a type of glue or cement/plaster material. Which would take somebody with greater knowledge to restore it then me.