Two companies build lawn tractors for Sears: Husqvarna and MTD. The Husqvarna machines are identified by the numbes 917, followed by a dot, followed by the model number - 917.xxxxx. I would select one made by Husqvarna, as I believe them to be better machines than those made by MTD.

It's very unlikely that it's an ONAN engine.

Not sure how you would recommend what brand to stay away from when you can't tell what brand it has.
Why would it matter if he runs them out of oil?

It's far more likely the engine is a Briggs, Kohler or Tecumseh.

my bad, I was tired, it is a 15.5 hp Kohler and it kicked a rod.