Your gonna need parts, so I would check out the motor schematics or a lawn mower diagram over at eReplacementParts.com - They even offer some mechanics that could help you out or get you headed in the right direction.
Brandon
December 2009
The crankshaft and camshaft gears are often made from plastic - or you could just have one plastic, one steel or both steel. Anyway, no matter. The smaller gear fits to the crankshaft, and its keyed in place. The larger gear (twice the size, in fact) is keyed onto the camshaft. Identify the timing marks - they may be dots, dashes or vee type on both gear wheels. Its vital that these line up when you do the rebuild or the valve timing will be out. Enjoy!
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lawn mowers.