The key way has sheared and its stuck together so how do I pull the flywheel off so I can change it

You will need a flywheel puller. You could concievably fabricate one but not worth the trouble. See web site below. Item # 363, Just example, not necessarily the correct one for your engine.

Look up engine model number and order from that part number.

Parts and Diagrams for Briggs and Stratton 10F902-0111-B1 (http://www.partstree.com/parts/?lc=briggs_and_stratton&mn=10F902-0111-B1&dn=56490007)

Hi,
The correct puller is #19203. See item #363 at:
http://www.odref.com/briggsandstratton/model-31/312700-ms5516-1102.pdf
Also from the previous question:
You must find a way to keep the engine from turning over and then usually with a 15/16" socket loosen the nut. If you do not have the needed puller, back the nut off up to the top of the shaft, take a pair of hammers(one a ball pein), place ball of the ball pein hammer on the tip of the shaft and strike the flat face of that hammer with the other, Something to carefully lever upwards pressure on the outside edge of the flywheel may prove of assistance. Usually one or two blows is sufficient. Do NOT strike the flywheel directly as that will break the stator magnet loose.
Peace,
Clarke