Can you cut off a section of the leakage outside the pipe joint of the classic Fox heater and reattach it?

You can't.

When replacing the heater tank in the vehicle, it is necessary to remove the workbench. In this case, the screws should be removed with professional tools before welding. The reason for the leakage of warm air water tank is that the metal pipeline leaks due to corrosion, welding cracking, vibration cracking and other reasons.