If you peek down from the inlet can you see any obvious rust or nastiness?

If you shine a torch in there you should be able to see if the cases are clean, or if there's any wildlife moved in.

You can at least check the con rod side clearences while you're there. It won't tell you the whole story but it's a good place to start. Also, grab the flywheel and give it a wiggle up and down. If you can feel it move then the main bearing could be a little dodgy.

If it looks reasonably good you could fill one crankcase with fuel/oil mix, give the crank a couple of slow turns and then leave it for half an hour or so to see if anything dribbles out through the seal. Then repeat with the other crankcase. If it leaks there's a chance that the crank seals are shot.

If you repeat this flushing a couple of times it shoul;d help clear out any debris in the cases.....maybe!