Zak- cheer up, man- as nasty as it looks in there, don't forget- it was RUNNING!
Flatheads get a little yucky due to shape, burn, etc., and sometimes they need a little de-carbonation. The guys' recommendation to run a little Marvel Mystery Oil in the gas every so often is part of that- it tends to act as a detergent or 'release agent' that knocks carbon loose and sends it barfing out the exhaust.
A similar tactic that was taught to me by an old leather-skinned guy that used to race Vincent motorcycles and Flathead Fords (what a combination!)... it was to take a Windex bottle, and fill it full of tap water.... start the engine, warm it up good, take off the air-cleaner, set the squirter to medium spray, flip the throttle open and squirt-like-heck... shoot it right into the throat, and as engine bogs, open throttle more.
Now, it's not the safest thing in the world, and there's better things one could do to an engine, but there's also much worse. By spraying water in there, the water (because of several physical conditions) breaks carbon soot free from stuff, and since the engine's running, it pukes it out the back.
I remember him doing this to a Flathead Ford-powered backup generator that'd been extracted from the basement of a broadcast station here in town. Apparently, it got to running really bad, and it got replaced for a newer type that was installed on the roof. even after it was restarted, it ran bad... until it got a healthy dose of water-injection. It rained carbon-crap for about 15 minutes afterward, and has been running like a champ (with a shot of MMO in every third tankfull) since then.
But since you've got the head off, scrape the worst off the piston top, valve areas, and valve tops with a putty knife, and suck up all the debris with a shop-vac, put it back together, and pour a shot-glass full'a MMO in the fuel tank.
Don't forget to rinse out the shot-glass before you put it back in dad's spirits-locker!