Can you repair a shower spigot bypass (diverter? What is the proper term?) pin that leaks when the shower is in operation,or do you just need to replace the whole fixture?

Since tub diverter spouts, (that's what to ask for) are so inexpensive just replace it. Be aware that there's two types of diverter spouts. Threaded and compression. To tell which one you have look under the spout next to the wall and look for a small square hole. If there's a hole then the spout is secured to the copper pipe by a Allen Set Screw recessed up in there. The material in such a spout,(compression) is chrome over high impact plastic. Threaded diverter spouts and metal, chrome over brass and must be unscrewed. Cheers, Tom