Question about Slant/Fin Sentinel hydrostat

Jersey2

I am thinking of purchasing a back up hydrostat for my Sentinel SE-140 boiler before prices go up. I looked up the part number- Hydrostat 46-1021-000. When I searched the web it says that this hydrostat is no longer made, there is one web site that has it but are asking a high price. Is there an equivalent hydrostat that replaces it?

I did see a hydrostat for the Sentinel SE-70, part number 46-1022-000 that looks exactly like the 46-1021-000 but without the onboard transformer, and has an extra wire harness. There are a lot of these on the web priced very reasonable. Could that hydrostat be used by adding a transformer? Below are what the two look like. What is the onboard transformer used for?