It looks slightly oily, but that is to be expected since it has been inhaling oil recently. It does not appear to have been inhaling much oil, just a whiff of it. That amount of carbon is quite minor - on the low side for such an old engine. The piston crown seems in good condition. The next potential problem to check is the piston rings, but they seldom fail suddenly. I suggest you rotate the engine so the piston is at the bottom of the stroke, and look at the cylinder bore. It can be in excellent shape if the rings are just worn out, but will probably show signs of scoring if the rings are broken. We need to find out as much as we can before considering taking the piston out to look at the rings, because that is a bit harder.
