As mentioned. Drain the oil, see what comes out. Better still, when drained, drop sump if owner will let you. Get a better look! Having said that, I bought a 750 Slabbie that had been left in the weather with plugs out, and carbs off. Inlets not plugged. I drained the oil and a bucket of water came out. Engine's fucked, I thought. On stripping it, bores were toast, bottom end was as new! So the one you're looking at may not be as bad as you think. Room for negotiation!