Sometimes in restoring an engine it is necessary to remove the flywheel from the crankshaft to replace a bad gear or to free up and clean or replace a stuck or damaged governor collar. Here are some of the methods I've used.
First, before attempting to remove the key, take a file and file off any burrs on the crankshaft. Also, remove any burrs in the key slot all the way to the end. If the flywheel key is a tapered gib type key and has a good square end (i.e. the shoulder on the end has not been beaten off), heat the flywheel hub with a torch then take a 1-inch wide chisel and drive it in between the key and the flywheel face. A few good hard blows with a heavy hammer should, if all goes well, loosen the key. Since a gib key is tapered, it loosens all the way when it moves.
If the end of the key breaks off before the key comes loose, the next step is the good old drill. Saw or file off the end of the key as flat as possible. Center punch the end, and with a drill bit slightly smaller than the key, drill all the way through the key. If possible remove the piston and rod, or tie rod up out of the way. Next, take a short piece of oak 2x4 and lay it in the base under the crank throw. Then bounce the crank throw against the block in alternate directions. Bounce the crankshaft throw against the wooden block as hard as possible. With perseverance, sweat, two cups of coffee, and a break for lunch, the hollow, drilled-out key will eventually collapse and can be removed.
Many engines, especially the larger ones, have a split hub flywheel and a square, untapered key. Remove the bolt from hub, then drive two chisels into the hub split (one at a time, of course). CAUTION: when driving the chisel, keep a good grip on it because the flywheel resistance to the chisel has a watermelon seed effect and the chisel can fly out with a lot of force.
After the chisels have spread the flywheel gap, rig up a wheel puller of your own design. Since standard wheel pullers are not very useful due to the six spokes, I have used a small hydraulic jack, some chain, and heavy steel bars. The square type keys, after hub is spread, have always (at least in my experience) slid off with the flywheel. NOTE: All of the above includes generous amounts of penetrating oil.