Hi Niels - Welcome to the nut-house. DETENT SPRING sounds the probable cause, though assume the worst and plan to do more when you open up the engine.
As you will need to remove the radiator - I'd suggest replacing all the radiator hoses (Samco Sport in my case).
If the engine has done a good number of kms - I'd consider replacing the clutch plates.
Also check the lifter piece and arm - they are almost certainly worn.