Heating Help

Need Help finding BTU Rating on Burnham Holiday

I have an old workhorse of a boiler- a Burnham Holiday. It runs, keeps up with the heating requirements needed in the Rockies, however I am wanting to price out a replacement system. The Boiler dimensions are 21.5"x24.5"x30.5", however I have searched high and low for a btu rating. If it helps, it has a Watts ET-30 Pressurized expansion tank attached to it. Our county is kicking in money for boiler replacement, and I would like to potentially take advantage of it. Thanks!