Hi all,
Little update. Since the fault was not found, i chose the way of buying another second hand engine and finally got to ride a couple of months end of last season.
Exhaust remained old one so all the possible crack doubts in exhaust were eliminated.
I have still not given up and took the engine completely apart again. It was no surprise that main bearings, big end bearings, pistons, connecting rods, crankshaft is fine. Also completely disassembled the cylinder head this time, valves, springs, valve split cotters all fine.
Yesterday came to mind a question relating to balancer shaft drive gear. Why is it designed with 2 separate gear wheels that are side to side each other ?
I can imagine purpose is to keep the balancer drive gear and crankshaft gear wheel tightly together ? Wondering if there could be a broken spring between the 2 balancer gear wheels.
P.S double click on the picture enlarges it

Many years ago, I used to service CNC equipment, and the measuring systems used the system as a backlash eliminator, so no discrepancy was measured when axis direction changed. I have seen it in many engines and systems over the years.
Its actually called a backlash gear in the service manual.