Replace the cable, using some 1mm mains rated cable, simply because the mains rated cable is easy to get in stores either as precut cable or simply by buying a standard 2m mains extension and cutting the plug and socket ends off it. Then the rear of the panel the connection box is easy to open and replace the existing cable using the new one, and feed it through the same rubber grommet, which will be a lot tighter than the original, and either the cable has screw connections inside or solder connections. You just use the brown and the blue conductors, and ignore the green yellow PE each side.
Makes a big difference in those panels, though I used 1m of the cable and then went to 4mm copper wire to run down, as I had a long way to go. On a good sunny day like today I get nearly the full 50W out of the panel.