Valves,Plug, regulator any could be a problem. Centralhobbies is a good call just as DD said.

Before you do though pull the valve cover off.
Turn the motor over till the valve's don't move for 1/8-1/4 of the prop revolution.
Take a dollar bill or a feeler gauge .05MM and slide it under both valves.
Adjust them so they are snug but not overly tight on the feeler gauge.
replace the Plug with an Enya or OS F.
Adjust the Regulator so it's flush with the housing.
Put a 12 X 7 or 13 X 6 APC Prop on it and crank it up. Should fire.