If you pit the best disc brakes against the best drum brakes then discs will win out every time. However, if you're building a race replica, don't discount the front drum option. While some use the Suzuki GT750 4LS front brake over here, the one to really get hold of is the old TZ Yamaha Fontana 4LS hub. It's enormous and when set up right it will stop you in race situations as good as any disc IN THE SAME RACE CLASS.
I ran a custom made cast iron disc, AP lockheed caliper and master cylinder on my CB450 racer and Dave Madsen-Mygdal would outbrake me any time he wanted on his CB450 with the Fontana hub. It's an awesome bit of kit and there are good modern reproductions available too from the likes of smith-kanrin etc. You'll have to dig deep though, Fontanas change hands for £1000-1500 depending on condition but you will have an awesome looking and working brake.
