The cam chain guide that pushes/lifts the cam chain is an item that can wear out, so replacing it is cheap insurance as you noted above. Something like $100.
My friend, without having the bike in front of him to see/hear, says these could
be possible noise causing factors:
If the owner installed the engine, the most likely cause is an exhaust leak at the #1 or #2 exhaust port gasket. Easy to do. Tighten down the nuts. If no go, then uninstall and reinstall the exhaust with new gaskets.
Otherwise, these would be my guesses:
-- Injector clicking (soft/fast, constant at all temps and is high on the motor);
-- Piston slap (louder, low on the motor, better with temp and proper oil level);
-- Piston-to-head contact (aftermarket pistons, worse with temp, low on motor, loud).
Note/Q: How do you top off your oil and site the oil level glass?
Hopefully it will just be something minor and easy to fix
Break her in nice... or break her in hard on a chasis dyno.