Is it a Raleigh Scorpio or a Scorpion. Scorpio is a 70's road/touring bike. I've seen pictures of a Scorpion MTB. Any MTB with caliper brakes I shy away from, it screams "low end".
Either way, Disk brakes require new hubs, which require new wheels or lacing new hubs into existing rims. Ditto with drum brakes. I see dual pivot caliper brakes as the lower cost alternative to the other two. Disk brakes also require disk brake mounts, which do not bolt on. Drum brakes have a reaction arm, which can clamp to the fork (bolt on)
Edit:
Also, steel rims or aluminum? Upgrading to aluminum will improve performance, but requires a new wheel or existing hubs laced into a new rim.