That will depend on how you’d like to approach the issue.
Simplest attempt, if indeed the issue is clogged rings.
Try adding Marvel Mystery oil to the crank case as per the instructions and see if it improves.
Might also do a compression test on each cylinder to see what the mechanical condition of the engine is in.
From there, do a leak down test on each cylinder to see if your dealing with worn piston rings and/or cylinder walls.