Just some thoughts -
I started losing power at 2500 miles on my first DAX. I pulled the piston, and the lower bearings were failing - the connecting rod wobbled quite a bit on the lower bearing.
Any of these motors I have pulled, the piston is always pretty loosey goosey on the top bearings, even on a new engine. You might want to take the pin out of the piston (remove clip on side) and check pin - is their a groove worn in it ? Are the bearings missing a roller ?
My first motor lost power at higher speeds , and I learned about the intake gasket. It must be air tight, these things will suck air like crazy. I have extras, but have had real good luck with one on my current motor (DAX) that I made out of a margarine box (that waxed cardboard) - I learned this from a post.
I have 3400 logged miles on my current DAX. I have never run it at full throttle though, and pretty much always pull the clutch and coast down hills - even small ones. My crusing speed is like 23 miles per hour. I got a 44 tooth sprocket, and use 20:1 oil that I get at Walmart by the gallon. This is my transportation, so I go easy on it.
Not that I don't want to go fast, but if you wind these things up going DOWN hill you are asking for trouble.
I am thinking your bearings are going, wiggle the connecting rod, should be tight and check the top ones. By the way, you can go to a chainsaw store and get Stihl 028 wrist pin bearings, these are supposed to work as upper bearing replacements - I have never had occassion to try them, but I bought them and they fit.
Otherwise:
1st engine I got 1500 miles, from LFM or some vendor like that - top bearings went and motor locked up
2nd engine from DAX, blew up (top bearings again) after 400 miles, DAX sent me replacement under warranty
3rd engine (DAX replacement) got 2500 miles (still running but big loss in power, bottom bearings going)
4th engine used from eBay for 60 bucks was real weak, I think the kid screwed up the ports with a dremel and wanted to unload it - I trashed it.
5th and current engine 3400 miles - seems to be running strong - a small loss in compression maybe over time, but not much. I put on an angle fire head for more juice.
I could rebuild the current one maybe, but the "aint broke don't fix it" applies to me
Your pics seem to indicate more wear on your motor than I would expect with that few miles. So again, I suspect the bearings.