Should I repair a damaged extension cord or just buy a new one?

Only repair heavy-duty extension cords that are hard to replace. For a cheap $5 hardware store cord, just buy a new one—it’s safer and usually cheaper than a good replacement plug. If you do repair, use a plug rated for the cord’s gauge and wire count, land wires correctly (white to silver, black to gold, green to green), and always tug test. Never splice a broken cord, skip the ground, or use tape instead of a proper clamp.