I have the proper double flaring tools myself, so i would just make up a new line.
For a temporary repair (likely only to last 20 years or so) is cut the line where the nick is, and install a compression union of the appropriate size. A less reliable fix would be to cut the line and install a short piece of rubber hose, but I would only envision 5 years out of that fix.
Could this have been your problem all the time? The pump would have been sucking air, and starving the carb for fuel.
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