Do you have a flywheel puller....if not, you can get a harmonic balancer puller from most auto stores for cheap. You'll need three 1/4"-20tpi (three inches long will do) bolts to attach it to the flywheel....use strong grade 8 bolts. You'll also need a rubber strap wrench to remove the flywheel nut (do this first) and to hold the flywheel in place while you crank the puller down. If it doesn't pop free when cranking it down, tap the puller with a rubber mallet. If it is stubborn, soak it with penetrating oil over night and try again. Make sure you remove the flywheel nut or you will damage the crankshaft threads. Also, do not apply heat to remove it if it is stubburn...that will warp the flywheel.
Once inside, inspect your coils. If the plastic coating is cracked, you'll need to replace them. Using very fine sandpaper or emery paper, clean the points until they are nice and shiney and regap them.
You can get parts at www.oldmercs.com. The WG4 is based on the Mercury KD4 and the ignition parts are interchangeable.