U,
You'll have to get a set of S&S rocker arms, and rocker boxes. :2thumbs:
Check the front cylinder lifter adjustment.
Check the front cylinder for oil usage.
Check front cylinder compression(release disconnected).
Check front cylinder leakdown.
All of the above may or may not point out a problem.
The ONLY SURE WAY is: to pull the heads, and check the valve guides for excessive wear.
I've been doing this for many years, on motors with less mileage than yours, and 10 out of 10 times the SURE WAY shows the problem.

John