Then namplate says 160,000-out /200,000-in = 80% steady state combustion efficiency.
At typical 2500' 2x4 framed house with R11-R13 fiberglass in the walls and R20-ish attic insulation and clear glass storms over wood sash single panes typically comes in at about 45-50,000 BTU/hr @ 0F if it has no foundation insulation,, 35-40,000 BTU/hr @0F if the foundation is insulated. The coldest locations in Iowa have a
99% outside design temp of about -10F, which means even 60,000BTU/hr of output would most likely have you covered, even without an insulated foundation/band-joist.
You don't need anything like 100K unless your walls leak as much air as fly swatters, and at a 160K of output it would probably keep up with several windows open even during sub-zero weather.
Being something like 3-4x oversized for the load means it wouldn't get better than 70-75% as-used AFUE efficiency even if it were brand new and actually had a flue damper (standard equipment on new boilers.) Without the flue damper you're probably looking at 65% AFUE if the thing were in "like new" condition, but probably 60% or lower given it's antiquity. Replacing it with a right-sized (or no more than 1.7x oversized) 83-85% efficency boiler would probably cut fuel use by at least 35%, and replacing it with a right sized condensing boiler maybe 50%.
The burners are build with little nozzles/jets on the manifold that inject gas in to the long tubes, that also have adjustable air intake, visible in picture #3. It's likely that the disfunctional burner's jet is clogged with either corrosion or crud (sometimes even tiny spider set up shop in them over the summer.) With both the power and the gas valve shut off the boiler you might see if you can't unscrew the jet from the manifold without ripping it all apart. Use the right sized non-wrench, not an adjustable.) The connection to the long burner tube is usually not a firm attachement- the tube simply slips off if you can get enough wiggle room. You may have to pull the manifold out wholly out to get at it, in which case you can probably clean up the jet without removing it from the manifold. When working on gas plumbing you need to use pipe-dope when re-fitting them, and it may even be technically illegal to do as a DIY unless you have a gas fitters license (as is the case in many states.)
It's always better to size the boiler to the load rather than to the radiation, but the lengths of the baseboard (on each zone) often affects the boiler choices. For 160,000 BTU/hr of boiler output to balance with the radiation takes over 250' of baseboard. Any less than that and it will cycle the burner during calls for heat. If you in fact have that much baseboard you would be able to take great advantage of condensing gas burners. If you don't have that much, it's critically important to limit the maximum size of the boiler to what the baseboard can actually deliver. But that's a limit, not an optimal size. The optimal size (per ASHRAE) is 1.4x the heat load at the 99% outside design temp, though it'll still meet it's nameplate AFUE numbers at 1.7x oversizing. Putting that into some perspective, assuming a 99% outside design temp of -5F, at 1.4x oversizing you'd be good down to -40F or cooler before it actually loses ground and can't keep up.
If you picked a new cast-iron boiler that was right sized for your load and put it out for competitive bid you can probably get it installed for $5-6K all-in, including a new system pump and a right-sized flue liner sized for the smaller boiler. For another grand you could get an indirect fired hot water heater to run as a separate zone, which would improve overall AFUE by giving the boiler a higher duty-cycle. Condensing boilers could be twice that, sometimes more, depending on the system particulars.