When an engine fails there is a reason, Do you know what made the rod snap ? a lack of oil can cause this. Or you may be over speeding them. or a combo of both. What kind Oil are you using you may not be getting a good lube if your using cheap oil or the wrong kind. Do you check your oil before your job? 10w 30 if its a overhead, If its not 30w would be good because they get hotter and need thicker oil.
All the Motors you listed are good. But if were not keeping up with them on a regular they will fail. I'm not saying your doing any of this, Just given you some common Info that i know and I've seen happen to people. Good luck.