How can I fix my lawnmower engine# mod. 124802, type 3436-01 briggs & strat?

I have a craftsman 6.0 Briggs & Stratton engine mod#124802, type#3436-01. While cutting one day it suddenly shut down while cutting.I was told the valve seats were not properly seating the valves. But they both look fine and I had no engine problems before now. How can I tell for sure if I have a valve problem of any kind? ( The engine turns but won't crank, the crankshaft turns freely by hand in both directions . Should it do this? Also could it be a broken crankshaft key? I think the ignition is fine. )