Well, the grand scheme of using a Honda GX390 was a bust. Great engine, just won't physically fit the way we need it to in this tractor.
Given that we are missing some parts from the K301 and we don't know what damage was done when the connecting rod came apart, I am now searching for a drop-in K301 or K321.
In the mean time, the 312 is hanging out in good company in my Dad's shop!