Pull the rod and look at the big end bushing. Make sure the bushing has not spun in the rod covering up the lubrication holes drilled into the big end journal. This is the usual culprit leading to rod failure in these type engines. Did it a gazillon times running Nitro boats years ago. At least until some of the motors started showing up with Needle bearing big ends. Then we started shearing crank pins, And the beat went on........

take care
Mike