Replace Piston? whats your opinion?

That's a lot of carbon for not "much hours" since replacement. If you put it back together with that piston, you are just going to spend more on gaskets in a few hours to replace it. I recommend measure bore carefully, replace piston now and correct any problems with the cylinder. Inspect plating near exhaust port carefully. But I've been in your shoes many times and "made do" until cash is available, so I get it.
But why did you tear it down??

That piston is scrap, but I have seen similar for sale on fleabay. Your bore seriously needs to be checked for being true. You have fine dust particles coming from someplace?

Not sure, but isn't there supposed to be an oil hole drilled in the piston right at that point of scuffing? Seems I've seen it in the wisco instructions (specific to honda)

TWRT does bring up a good point, for 1 issue. All the Hondas I have, does have an exhaust bridge, and does have 2 holes drilled. But this and the missing o-ring did not scar the piston, except where the holes should be. This is 1 place, looking at something gets into an opinion? If you see a difference, I call it wear, and worn out. I can barely feel it with a finger nail, is huge on a part with .002" tolerance. That missing o-ring will make a huge difference in how it runs and vibrates. Exactly how long is a crap shoot, a heavy throttle person could blow a loose engine in no time. This is where it can get dangerous for the operator, one may never know exactly. It is/was a 2 stroke MX race engine, not a trail bike. You have to measure, and being precise does matter.

That piston is well past a few hours old! No question.....replace it. Have that bore inspected by a professional as well.

Could possibly be one or more: worn beyond specs, out of round, flaking nikasil, cracked exhaust bridge, improper relief of exhaust bridge. Or everything may be fine and dandy. Won't know until you do close inspection and measurements throughout the bore. If this were a piston with 30 hours on it, I wouldn't worry as much about its condition as a piston with not very little time as you were led to believe. My suspicion is that it has many more hours than you think.

Have you noticed any drop in the tranny fluid? A bad right crank seal will build up metallic clutch debris, and cause seizure at the exhaust bridge. Actually I think its because the bridge gets so hot, the aluminum debris melts, and adheres to the bridge, and builds up. Normally the bridge has extra clearance, so its expansion does not seize your rings and piston. The bridge and bore will build up flashing from the piston rubbing off. A normal rebuild may show the cross hatch marks gone, in reality it is under the aluminum flashing, removed with oven cleaner. Using a ball hone is terrible in the wrong hands. The bore, running long on rebuilds actually pushes the softer material behind the plating out of round. .0035"~.004" is not much, yet all the difference it takes to make the motor weak and spooge. I do not believe the sleeves are prone to get pushed out of round as easy, they just eat top ends quicker. And of course, ANY amount of foreign material, speeds this process up. From very fine soot to actual pieces of sand, it will LOOK good to some, but just by SEEING it tells its been wearing out quickly.