I recently purchased a reproduction temp gauge kit for my 56 f100.  However the temp sending unit was missing.  I have tried to reach out to Mid-Fifty but haven't gotten a response yet.  Would anyone know what type of temp sender I would need to complete the setup i.e. part number and manufacturer.  

I'm not familiar with Mid-Fifty, but I would continue to try to reach out to the seller. If you're not successful, find a vendor that sells the same gauge and purchase a sender that is compatible with that specific gauge. I'm finding that many/most 55-57 Tbird reproduction parts come from one or two manufacturers but are sold by many vendors. Many/most of those parts are made in China and India.

I'm sorry to say it, but there's a good chance you won't hear from Mid Fifty, considering they're going out of business.  

Thanks 55blacktie I finally got a response from Mid-Fifty and was able to get a part number for the temp sender.  Although it was supposed to be part of the kit it was missing and they had none to supply.  Kit was deeply discounted so I didn't put up too much of a fight.  Shame that Mid-Fifty is going out of business. 

https://midfifty.com/item.php?INV_ID=5116&PN=Temperature-gauge-and-sender-kit-with-1-8-npt-sender-mf
I guess you've already seen this. I'm just too slow in my old age.
stuey