Unconventional thermostat wire labels on boiler
Easy answer: use a relay

Since your heater expects a contact-closure across its TH terminals, and doesn't document what voltage it provides there, I would use a garden-variety SPST-NO 24VAC fan relay to adapt a modern thermostat to this heater.  It can be mounted at the heater, with its contact terminals connected to the TH terminals on the heater with short lengths of 18AWG wire and female crimp quick-connects, one coil terminal connected to the C terminal in the same fashion, and the other coil terminal connected to the white wire in a new thermostat cable run up to the thermostat location, using another crimp quick-connect terminal.  

You then can connect the red wire in the thermostat cable to the 24VAC (R) wire on the heater and the blue (or green, if you're using 3-wire cable) wire in the thermostat cable to the C terminal on the heater; this gives you roughly 12-15VA of 24VAC to run the thermostat with, which is enough for a basic programmable thermostat.