Don't quit now- we're just getting started on the sizing issue!
The Sentinal 105 SE has 87,000 BTU/hr of output, which is something like 2x the amount of heat needed for most homes, and it could easily be 3x oversized for a smaller better insulated house. You only get a shot at right-sizing the boiler every 20-30 years or so, so while investigating, why not start with analyzing your heat load to avoid oversizing, which has both efficiency & comfort consequences. Unless you're convinced the next ice age is imminent and you'll be seeing -100F temps every winter, you're probably better off with a smaller boiler more appropriately sized for the load. It's very common to find older boilers 2x, 3x, even 5x oversized for the actual heating loads, and relatively rare to find them less than 1.7x oversized (which is the oversizing presumption in an AFUE test.) Just because the last installer put a 105,000 BTU/hr boiler in this place doesn't mean you need to repeat the mistake (and it almost always IS a mistake to install a boiler that big, unless it's an unsually large &/or uninsulated house.)
If you have fuel-use history on the place it's possible to get a pretty good ball-park on sizing by calculating the fuel-use per heating degree-days, and calculating how much you would need to cover the load at your
99% outside design temp. All the necessary information can be found on a mid-winter gas bill, with the EXACT DATES between meter readings, plus a zip code (so we can look up the weather history for that interval, and estimate the design temp), and both the input & output BTU ratings of the old boiler.
But if you've already uncrated the thing and started plumbing on it you probably can't take it back for the right-sized version. :-( Within the same model line the SE-70 (smallest in the series) is more than enough boiler for the vast majority of homes in the US.