Check the white wire,it probably goes to the safety valve ( and a wire between the flame switch and valve as well) I don't think Monarch is still around so wiring diagram would be hard to find. Try a meter and test the old contacts. Seems like 4 would be L1, 5 would be common, 2 indicator light.( on new Harper Wyman tstat) When you stated the"thermostat broke" what broke? no heat,too hot, low heat?