Small Engines (Lawn Mowers, etc.): Kohler 18HP engine wont turn over, kohler engine, fan blades

Question
I do believe that my Kohler 18.5HP Magnum (MV18S-58560) in my Craftsman mower may have seized.  It ran for a bit, then died, and now the starter (which seems good) can't turn it.  I've tried turning it with my hands, by the fan blades on the flywheel, but it won't budge.  

How do I determine if it's fatal?  Can I rebuild it?

Answer
oh  boy !!!!!     remove the plug.   make sure there is no  belts  or anything holding engine back.   make sure starter gears did not jam...  if there is nothing holding engine back,  starter is fine???   and you still can not turn it  over???   i,d say  then it froze up  by lack of lube, or something letting go.  does this one have an exturnal  oil  filter?   if so   may i let you know,  that they get dirty quick,  have no bypass system in the filters,   and when they get plugged up????   they do not lube the engine at alllllll !!!   bad set up !!!   being a kohler engine>>>  yes it is worth rebuilding, but must come off and apart to determine  what is needed.