If you can find the installation instructions, either on-line or if you're lucky, you have it; it will define the surface area of the air supply requirements. WIthout a forced fan, a pvc pipe probably won't cut it. The best way may be to build a room around the unit - leave plenty of room to work on the thing and a big enough door to replace it when the time comes. Then, run a duct to the outside into the room of sufficient size to allow it to run properly into that new room. There are some general rules based on the BTU output and efficiency, but it's best to use what the manufacturer says. There should be no reason to readjust the burner, but it wouldn't hurt to check it; it could be off after all these years.