My guess is the mag bike with front and back is a 78-E model? The 80 only was a front disc bike. The entire wheel and forks ( or just the lowers) would have to be swapped for the the front. The entire rear wheel and swing arm would have to be done as well as all the 78-E mounting brackets welded to the 78-2E frame. It can be done but it will add a lot of weight and disc setups are a lot more work and maintenance than drum and cable setups.