Is it possible to keep the standard forks off a cross country bike, currently holding cantilevers and fit discs, or do I need wider forks?
Does the fork have the disc tabs on the left leg
There is more to it than just mounting tabs. Disc brakes put much different loads on the fork legs and different forces on the hub axle/qr skewer. The fork legs have to be designed to handle the torque and the dropouts angled properly to prevent wheel "ejection". You don't just braze disc mounts onto a fork meant for canti's or V-brakes.