Have you looked at your head closely for interferance with piston / valves.
Dropped valve guide or something.
I had mine start a ticking noise.
When it was torn down it was obvious it was the valve seat.
However the guide had moved also.
Go back and look at your old piston for top surface damage very closely.
Mine was showing signs of lower compression when I was starting it. This was about 10,000 after a major rebulid.
We kero tested the head at the rebuild and it was fine,
We had the valves reseated at the time, came back border line for thickness but we used it.
Now have repalced with new head, valves and some rockers, cam was whole new cam at rebuild time so did not need work.
Did you also check the oil holes in the crank before you reassembled.
Did you pump oil in the crank shaft end oil hole and see oil coming out of the connecting rod big end.
You could have a blocked crank oil hole.
Did you check the oil screen in the bottom of the frame oil tube to make sure it was not blocked with the teflon / nylon from the cam chain guides as they wear.
Biggest cause of engine oil starvation in XR's after not feeding them enough oil.
Hope you get it sorted.
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2005 BMW K1200S
1997 Honda XR600R