Need to understand what fuel you'd use: NG, propane, oil, other?
Then, do NOT just replace with one the same size...it is highly likely that the one you have there now is 2-3x bigger than needed, and thus, is not giving the efficiency it was designed or capable of when new, let alone now. Then, check with the local utility company as there may be rebates that would only apply to certain brands and/or efficiency levels.
So you can compare apples and apples, you need to either pay for or approximate a reliable heat loss calculation. If you have or can get last year's energy bills, there are ways to use those for an estimate, otherwise, you may need to pay someone to run the numbers.