Small Engines (Lawn Mowers, etc.): Dyt4000 18.5 briggs, briggs and stratton, minimal modification

Question
I have a Craftsman DYT4000 (917.2736403) with an 18.5 Briggs and stratton(31P777-0348-E1)  The motor doesn't run.  The rockers are loose, when I stick a screwdriver in the sparkplug hole the piston isn't moving with the flywheel.  The rockers move with the flywheel.  When I turn it over it makes a clanging sound.  The flywheel key is fine.  I'm guessing it's a broken rod.  What do you think.  I'm going to tear it down when I get some free time, just thought I would ask an expert if there is a common problem with these engines that I'm missing.  Is it worth fixing these 18.5 briggs engines?  I read that they are junk.  The engine has 70hours on it.  The plug is black and gritty as well.  I also have a 17.5 Kohler engine in my other tractor that runs perfect.  If fixing isn't an option could I transplant the Kohler into the Briggs location with minimal modification?  Thanks in advance.
-Mike

Answer
There are no common problems to this engine that I know of.  About 7 years ago there was a problem with bad camshafts, but if your engine is newer this should not be a problem.  If it had a bad camshaft you would not be able to start it.

you are correct in your assumption of a broken connecting rod.

You can not transplant a kohler inot the location without lots of modification.  ie wiring, muffler, air intake, etc