Just meant if you can see any light past the rings in line with mainly the exhaust port then that'll be where compression is escaping as rings do wear inline with the ports or exhaust port being the biggest one. But that's more related to a high performance motor with big ports so in hindsight not that relevant on yours. I could tell you straight away if compression is good by feel on the Kickstart in reference to a faulty comp gauge reading that is. I must concede Mark is correct & to leave a seize mark it must be growing front & back to get to the point of seizing so would leave marks both front & rear of the skirt. Hope she goes good with the Wosner kit & rebore. Be sure to have the ports chamfer top & bottom so piston & rings can pass by smoothly. I'll just add on rethinking just my thoughts but it seems odd for dirt to attack just one area of the piston & dirt also needs to go down into the crankcase then gets sent up the transfers to the top with the fuel mix so has me wondering how the conrod & bigend feels now you have the cylinder off. If only motors could talk!!. Although probably best mine don't talk lol.

I would not powdercoat engine parts for the reason you already mentioned...  the thick coating inhibits heat transfer. 

Here is one article that talks about compression gauges.  There are (or at least were) YouTube videos as well where people have demonstrated the problem.  www.klemmvintage.com/squish&comp.htm

"Tuning filter that doesn't need oil" sounds like it's going the wrong way.  Many performance filters (even K&N oiled gauze) allow fine dust to enter the engine.  I'd use new foam and Twin Air "Liquid Power" filter oiling system.  I've used it since the 1970's and the Twin Air product works awesome.  Catches the dirt and is very easy to oil filter correctly.  It's very thin, almost like water, when you apply it.  Then you just squeeze out excess and let it dry overnight. 

I will look for the bulletin on the air box seam defect so you can check yours.  I think it was mostly the earlier bikes (1974) in that series that may have had the issue but it can't hurt to check. 

A friend messaged me and said the intake-side-only scoring can also happen if engine is starved for oil...  like after first startup when sitting for years.  Or after running pump dry.  Probably plenty of oil in bottom end but straight gas washes down intake skirt on reed valve models first.  Once pump starts delivering, problem goes away. 

I always use premix for first start on a bike that has been sitting a long time so I've never seen that happen...  learn something new all the time.

I'd check the stroke clearance on your oil pump .
If your clearance is under spec , or non existent at idle setting , the motor will starve for oil at idle ,
and deliver less than it may need at faster speeds . The seizures tend to be on the intake skirt ,
rather than on the exhaust side . That may not be the issue in your case , but it would be worthwhile checking .
A new piston may wind up the same way if that is the problem .

Mark could this also be caused by using Starting Fluid as was always told not to use it on outboards?

It's possible that starting fluid might wash off oil?  I think the stuff you buy these days has some sort of "upper cylinder" lubricant in it.

What I saw happen once with starting fluid was a lot was used, big backfire, and blew out a crank seal. 

Piston top is a shadow from stud

score marks are 4 point scuffing from cold running at high rpms

That was 50 years ago and starting fluid  probably has change since then.

Hey just a quick question: Markt said, he would recommend the powder coating beacuse of heat problems.
What does the rest say? I will ship the engine parts today for powder coating, but im not sure if i should powdercoat the cylinder and cylinder head.
I found a shop which has a special high temperature powder for engine parts. But i dont know if the engine parts are originally painted/powdered.

I will comment the other points later :-)

In post 12 MarkT said he wouldn’t powder coat the engine and I agree