Boiler, High Carbon monoxide levels, need info ...

The boiler may not be getting enough air but its almost a sure thing that the CO measurement was taken with the door wide open. I suggest three things on the to do list. First, someone needs to make sure the chimney is clear and measure the draft. Second, since it looks like no one has ever cleaned this boiler, the burners need to be pulled out and cleaned. This may require some compressed air and brushes to clean the sections above the burners and will require a vacuume to clean up the debris. While the burners are off, I would spray a bit of carburetor cleaner at each orifice. Third, after re-assembling everything, the firing rate needs to be measured either by measuring the gas pressure or by timing the gas meter. I personally would derate this little boiler 10% and then test for CO before dilution air is introduced.
While this boiler is not very efficient, it can be made to burn the gas completely and safely. The difficulty you will have is finding someone to do good work. No one in the past has done good work. If you don’t want to leave the door open you could undercut the door another 2 or 3 inches.