Small Engines (Lawn Mowers, etc.): Craftsman rider mower, weight oil, oil leak

Question
I have a Craftsman rider 917.273140.  Kohler 170 OHV.  I have an oil leak coming from the air cleaner cover that leaks onto the exhaust.  There wasn't a gasket but everyone i talk to tells me the gasket went bad.  There was never a gasket.  How do i get this to stop leaking?  Last leak caught the mower on fire.

Thanks for your time

Answer
Can you make a gasket?  

http://www.kohlerengines.com/onlinecatalog/pdf/tp_2339_d.pdf

Have you checked the Crankcase Vacuum to make sure the engine is maintaining vacuum?  If the engine does not maintain vacuum an oil leak can occur.

What weight oil are you using?  Does the oil smell like gas?

Eric