Start with a compression test on all 4 cylinders. Inspect the plugs as you test. The dead cylinder should be obvious. The issue may be either a defective plug wire or corrosion on the coil. The plug wire spark plug end has "guts" to it that can go bad, it's a spring and a round brass piece and a cap that unscrews.
With the '84 only there was an issue with cracking of the frame on the mount for the rear shock. Make sure you inspect for this. Also, the TCI is particular for that year only. It has to do with where the vacuum advance is taken from. it is exactly opposite phase from the other years, and affects acceleration.
Once you get the bike running on all 4 cylinders we will be glad to help you out with additional issues you may encounter. As far as delinking the brakes, that is a personal preference issue. The '83 to '85 brakes in general left a lot to be desired; the brakes were improved starting with the '86 model. Stainless steel lines help out the braking...