Ever the optimist and self styled skinflint, here's my thoughts.
Couple of maybe unrelated events, or combination of several factors.
Sump full of oil, despite a good return and a top class pump. Engine could have wet sumped on standing, and with faith in that new pump etc not checked. Sure enough out of the breather it comes and onto the crankcase, simulating a major failure.
The cracking noise and metal bits are a conundrum, one looks like a piece of alloy, the other a tinge of yellow, both non magnetic ? So how to proceed?
Forget the oilseal for now, it can be easily replaced once the cush assembly is off.
As for the metal, a looky up into the crankcase via the open sump. Look for anything out of place on the flywheel rim, marks, gouges, witness marks on conrods, big end nuts. With the plugs out turn the engine by hand feeling for tight spots, strange noises, unexpected resistance. For the sake of a couple of gaskets, a full check of the timing side is worth doing if nothing in the crankcase is amiss. They may be unrelated to what has happened and been there for a long time. Do they look "fresh" with a bright surface indicating recent damage, or well carboned up?
The take off for the rocker feed should come after the filter, otherwise the return will take the easier path via the rockers and fill the sump rather than the tank. I doubt yours is incorrectly fitted, but worth a check.
Finally....it's a simple twin cylinder motorcycle engine. Reach for a beer, count your blessings, it's an easy fix.....compared to some vehicles featured in this Forum.
Swarfy.
