It takes two seconds worth of hand soldering using an iron bought from RadioShack to replace cartridge batteries. Replacing batteries inside Pokemon cartridges, especially Gold and Silver, has been a big thing for several years now, and really the only way that these games can continue to be sold on the second hand market. I see advertisements all the time from local used video game shops that say they'll replace your batteries for you for a small fee.
I'd be willing to bet you could even replace most of the capacitors and resistors inside cartridges with relative ease, the parts are probably on the internet somewhere. The only part that I think might be troublesome is the ic chip containing the ROM, since after replacement you'd have to reprogram it, but even then that seems like something that is more than possible.