I wish you luck on this one. Not sure what is up though. Our house is New England 1975. Same heating setup. And it’s 24v. We do have a 120v thermostat but that’s for electric baseboard heat in a bonus room.

So when whoever installed this originally. I’m surprised they went 120v

For us. The entire furnace uses 24V AC to do what it needs to do. There is a converter to go from 120->24V. Looks like a doorbell transformer. If you have one of those, you could short the thermostat 120v line and use the 24v for the thermostat like a normal hvac would work.

But, there’s a reason someone ran the thermostat at 120V. It could be laziness, but for brand new installs that’s not super common.

Thermostats are light switches. They will connect two wires when they want heat. Or disconnect them when they don’t. So long as you have that, you should be good.

Overall, can’t help without a lot more info / wiring diagrams. But it may be easier to call someone to handle the conversion. At the end, you want at least 3 wires. The two for the 24v AC, and a third to send the on/off signal back to the boiler.