1973 4/6 cyl Merc ign components will interchange with your '77 IL6. Maybe even a 1972 IL 4/6, but you will need to look at the redband and see what it has.
Those older Mercs had the orange coil and the big rectifier with the 3 colors on it. The switchbox was identical, and I think that is the key to compatibility. if it is the same, the components should switch over. The distributor on the early models may have a different exterior casing, but the internals were the same as the '77. You may need to swap them over.
Obviously the wires on the distributor are often bad.
