Valve guide migration?

Hi folks, 
 
This is my first time attempting a home repair of my equipment, help is much appreciated -- I hope this is an appropriate space to ask for help about my snowblower? I have an old Ariens Deluxe 30 snow blower that my father recently gave me but it doesn't run. Have tune up parts on order (carb, spark plug, oil etc.) but when he gave it to me he explained that it "backfired" one day and he broke the recoil attempting to turn it over. After pulling the cylinder head I saw the push rods on both the intake and exhaust were bent. After doing some research online I see that valve guide migration is often a problem with these old 21M314-2121-F1 briggs engines. I saw a video on youtube where they tapped the guides back in and drilled a hole through the head into the guide, threaded it and placed a set screw with threadlocker. I plan to do this as well but it's not obvious to me exactly how much my valves have migrated. Does anyone know how far above the cylinder head the guides normally protrude? The cylinder head also sticks up around the exhaust guide by design (something I haven't seen online) so I'm not sure whether to measure from the top of that or the floor of the valve cover. 
 
Your help is appreciated :)
 
Seth