How deep the scored/scuffed lines normally are? like 0.005"?
Well scoring of the piston skirts doesn't necessarily guarantee death is here, ring seal is the most important metric of all b/c heat of gasses leaking past the piston rings can cause the lubricating oil to fail and once the lubricating oil cooks onto the side of the piston the chance of seizure increases, probably exponentially. If the oil film fails due to heat escaping from the combustion chamber and cokes the oil to the piston skirt then the clearance between the piston and cylinder is reduced at the same time as the oil film is compromised, combustion heat absorbed into the piston crown normally travels through the oil film and rings to be absorbed out into the iron cylinder walls.
Any little thing that increases friction can throw off this thermal balance to a point where no oil can continue providing a lubricating film and combustion gasses sneaking past the rings can cause the sides of the piston to become soft and at risk of thermal runaway. Throw in a thermal runaway event and the side of the piston becomes molten, blows out and smears on the inside of the cylinder quickly causing seizure (molten piston sticks to cylinder).
This (seizure) will eventually happen to every 2-stroke engine as the ability of the rings to prevent combustion gasses from escaping decays over a period of time, hundreds of hours of operation in some cases depending on the thermal capabilities, construction method, amount of heat generated (by running hard) and overall design of the engine. The 951 is quite the Ferrari of small 2-strokes, IMO.
In the case of lower performance engines such as small antique outboards (951 doesn't qualify) I have on occasion removed fairly scored pistons and cleaned them up using solvent and sandpaper, used muriatic acid to dissolve the aluminum dotted up and down inside the cylinder walls and installed new rings with a light hone to the cylinders using emery cloth.
I guess the point is, IF you experience a ring or piston seizure due to a heat related failure there's a good chance you may find pieces of broken piston begin bouncing around in the crankcase, and from there the damage possibly snowballs into a broken piston rod liable to punch holes through the engine case. But, so what?
I don't know what that coking is I see on your piston skirt but if that's oil varnish and were to begin decomposing due to heat it could "ball up" and cause the piston to bind in the cylinder.
Scoring of the piston along with coking of the oil and low compression are all indicators of the eventual root causes of failure every 2-stroke will naturally experience, it's only a question of when, not if, and the only way to avoid this is by rebuilding before it happens.
Unfortunately the cases are expensive, in good condition possibly worth up to $500?
On the plus side, now you're using XPS-2 there's a slim chance some of that coking may actually be cleaned away, but I wouldn't hold my breath waiting for that to happen.