There are a lot of unknowns here so I'll go with some general 'ideas/thoughts'.
IMO, any natural gas unit should have fresh air into it and 'spent' air vented to the outside. Otherwise, you have a hazardous situation. If you're NOT getting outside air now, you do want to get it. If you're NOT venting your unit to the outside, you DO want to do so. (The only exception would be a ventless heater. They're built for that.)
With this in mind, you an use PVC that's about 2" in diameter and run through the wall to the outside. Once it's outside, finish it off with a 'T'. In the room, try to get the piping NEAR the unit but not in a place where air may blow out your pilot. If the unit is built to 'take' air in from the outside, then hook it up. (The owner's manual should speak of it. Call the mfgr. if you DON'T have one.) Otherwise, again, terminate it near the unit's intake.
If you have a service warranty, you can have an inspection done on it and I'm sure they'll tell you you're out of compliance. A Home Inspection should reveal the same thing. Don't think you're fixing your problem by hanging up the old door. My best to ya and hope this helps.