Removing carbon from heads and pistons.

My advise
Both intake and exhaust closed
Remove spark plug
fill chamber with liquid sea foam from can
Let set 48 hours
Add Sea foam to gas in tank
check for sea foam in cylinder first, then start and run engine until exhaust is clean
Change oil

Mist a gallon of warmed distilled H20 into the intake with a finger on the throttle to keep engine from stalling.

Did you ever see the cylinder on an engine whose head gasket leaked coolant into the cylinder? Not a speck of carbon to be found.

Google 'water decarbonization.' No need to bust out tools or wire wheels or oven cleaner.

Or you can run some techron or Gumout Regane, or some Redline Sl-1 or some berrymans b-12, or Similar PEA based fuel system cleaner through some fuel for a slower clean.

If you have the heads off (sounds like you do), spray them with Easy-Off Oven Cleaner, let them set for several hours, respray if necessary (you'll be able to tell if the carbon is coming loose), hose it off with water and blow it dry with an air compressor...make sure you wear safety glasses and rubber gloves.

A small amount of water injection will do it while running under load.
Otherwise scrape or wire wheel away.
It is not as critical as you might think as long as you are reasonably careful.

Manufacturers usually recommended a wood device so as not to gouge. I always used a wire wheel on the heads. I would just scrape the piston so you dont get a piece of wire brush in the gap at the piston to the cylinder. Its best to run full throttle when mowing.

Wire wheel on a bench grinder, or a drill motor with a wire brush that fits the chuck. Please wear goggles.

No easy way I know of, though others might.

Only effective way I know of is the old hammer and chisel. And yes, takes a great deal of care not to gouge around on the aluminum. Good luck.