Not a failure of the engines, a failure of the owners to do proper maintenance.
They usually come to me in desperation after having replace the battery, starter , PTO switch key switch , solenoid .then the oil & filter as if would make a difference to cranking.
I used to adjust the valve lash , charge the $ 15 for the 1/4 hour of labour and send them back out the gate.
What killed me was a crappy attitude like it was my fault they were so stupid.
So now days I tell them it will take at least a week. giver the mower a good going over, adjust the valves and send it back in 1 to 2 weeks charging them 3 hours of labour ( usual full service charge ) tell them it had me beat for ages .
When I do that they thank me greatly and choof off happy as a pig in mud