I have a Murray rear engine w/ 12 hp B&S.  It needs a valve job anyway so I'm hoping to get some more hp and torque as I do the repair and as a nookie I have a few questions:  1)  Can I replace the stock valves with larger diameter?  2)  Is it possible to do a 3 angle grind on the valves and does it do any good.  3)  After porting the "eyebrows" I'm going to lose some compression.  Is there an available "how to" about welding material to the roof of the cylinder head to raise compression?  (And yes I did search before posting my questions)

This always starts the same way... something breaks and I can't seem to put it back together without trying to get more power out of the engine.  What I have is an 11- 12 year old Murray Select 30" rear engine mower with a 12 hp Briggs & Stratton engine.  I may not get into racing but the simplicity of this B & S engine appeals to me.  I've tried to find go fast parts by googling Briggs & Stratton 12 hp model # 283707 but I honestly haven't been able to find the kind of parts I want.  Is there nothing for my old B & S engine?