The slinger throws the oil off to the side of the little cavity it is in. Then it is sucked back down to the intake manifold by drillings in the crankcase and reed plate. As Chinewalker said though, leakage may overwhelm the system if the bearings are loose. Otherwise, clean everything up, including the mag plate mounting rings and proceed. If you have doubt about the drain passage being open, pull the reed plate and blow compressed air through the drilling.

And don’t over lubricate the mounting rings. Excess grease will only splatter out and foul the points.

I use brake parts cleaner from the auto parts store. Cheap when Advance Auto has it on sale BOGO.

OK, well the plastic is probably left over from an older coil that split apart. Kinda hard to know exactly what is causing the oil residue, could have been from improper/over oiling the mag plate during a past servicing. Or, could be coming from the upper crank/slinger system. Have you had the powerhead off to do a water pump job? If so, was the lower seal assy properly positioned with good oring/gasket when you disassembled it? I’m not quite sure just how this system is "plumbed" into the crankcase, or what provides the low pressure to pull the excess oily stuff down and away from the slinger.
For now, it is probably best to clean everything up and retry the engine. Remove the flywheel again after running the engine for a month or so to see if the excess residue has returned, and go from there.