I would second a firestrike being your first mod. Cheap, easy to see results and can dive into a variety of mod work, like spring, barrel, plunger, putty and even simple electrical.
OutofDarts probably has the best data on hop up wear, as he's probably shot a sh*t load of balls thru Zeus and he's the only person I've heard say they wear out. He probably has an idea of how to replace them, but I can't see there be any money in that since its just a piece of foam probably the size of half a postage stamp. I can't take mine out, but it feels something like a soft rubber flap. Much firmer than silicone, but softer than a rubber washer.
I'd bet a suitable replacement could be found in either a craft store or the hardware store.