Two years ago I had a problem with the Briggs ELS engine on my 2008 Scag Tiger Cub and I repowered it with a Briggs Commercial Turf series engine. At almost 300 hours, I am very pleased with this engine, particularly with the Cyclonic Air Filtration system as I use a collection system most of the time. The new Vanguard series engines are an upgrade of the Commercial Turf engine and now Briggs offers EFI and several larger displacement models.
I also have a 2012 Hustler X-ONE with a Kawasaki FX730V engine, nice engine, but more expensive.
For me it would be a tough decision between a Kawasaki FX engine and the Vanguard EFI engine. My next machine might be a Ferris, but that is down the road!!!
If dust is a problem where you mow, then an engine with the two stage Donaldson air filter would be my first priority. ZTRs are dusty machines by their nature and good air filtration is a top concern.