kawi or Vanguard. They are both very good reliable engines overall. I know kawi's have an engine or two that are kind of problematic. I believe their FR691 engine series is one that has some pretty common issues and Vanguard probably has one in there too.
If you are going to buy an engine, buy the commercial series engine. Even Kohlers can be good, the Command is pretty well regarded while the courage is complete and utter garbage.
Briggs has a "commercial series" I believe, but the Vanguard is still a large step up and built for the long haul.
I have a rider bought new in 1995 with a vanguard 16 HP it now has over 1500 hours with nothing but basic maintenance, oil, filters, plugs, and one valve cover gasket. I also should add that I should have opted for the 18 HP model at the time because with a 48" high suction deck, that 16 HP gets worked hard.