I have A hustler super mini z with the honda 24hp engine. I think it is great. I've have had no problems at all. I think honda will take the market over in a few years. also think about this honda makes the best engines in a lot of area's (compact car's, generators, outboard motors, construction equinipment) I don't think they would stop now.
You should be able to install a Kohler or Kawasaki on it. It might be necessary to drill new mounting holes if the holes don't line up, but that is no big deal. If it's a 60 inch deck I would install at least 25 HP and 27 would be even better.
Check with DC to see what engines were offered on that year and model.
As far as engine life is concerned, if the engine has been well maintained it will last 2500-3000 hours before needing a rebuild or replacement. Most 4 stroke engines will begin to lose power after about 1500 hours of use and will continue to lose power gradually after that. I had a 23 Kawasaki with 2600 hours on it before rebuilding it. When I had the engine checked on a dyno at a dealer in MI it was producing 18.5 HP at that time. After rebuilding it and installing a new cylinder head and uping the compression ratio a bit it was producing 24.1. I kept the mower for another 2 years and 1600 hours before selling it.
The only thing I know for sure about Honda 4 stroke engines is that they have lower torque ratings than Kawasaki on comparable HP engines. I've heard other LCO's say they wish that they would have got a Kawi or Kohler instead. I'm not slamming Honda engines. They make damn nice outboards. But I've never owned a mower with Honda power so I can't speak from experience.