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@FredJ, Hvacnut was correct as there are 2 available openings in the bottom of the air box for the fuel lines to be run through. There are grommets available for both openings to seal the airbox from the boiler room. I have attached a sheet to this post showing examples of this for you to review. It appears that the at one time the bottom of your air box was altered to accommodate the hard piped fuel lines to the pump. The air leaking into the box while not being desirable should not cause an odors issue. This is most likely being caused by the oil leaks you mentioned in your post. I would recommend the replacement of the hard pipe with flexible oil lines at which time you could seal the opening at the bottom of the box with gasketing material, a full combustion test and making sure you have the pre and post purge functions enabled on the burner. As long as there are no oil leaks, the burner is adjusted correctly, and the chimney is operating at negative pressure there should be no odors. For the best result I would also consider eliminating the return line to the fuel tank and installing a fuel oil deaerator.