Try
www.dd-wrt.com, they have a pretty good forum and wiki section for various routers, you may find a way to flash it back to the original firmware. Anyway, once you see the extended capabilities of these open source firmwares, you'll be very unlikely to even think that reflashing back to original is a possibility! I have been running dd-wrt on my Linksys router and now I put it on every router that I get my hands on, without even bothering to set up the original firmware!!