While I see that long term 4 strokes will be “the future” it may take a while for that to occur. As a relative newby (I got my kart July 2006) I suggest that you look for a good used kart to get started. I’ve seen a couple of good deals posted on this site for not much more than a new chassis alone. That could get you started and well on your way having fun while you decide on the details of 4 vs 2 cycles after several times out at the tracks. By then you could decide to stick with a 2 cycle or buy a new engine 4 or 2 cycle, while selling off the old engine.
Personally, and I could be wrong, engines need a re-build based on how hard and hot they are run. If you are not racing or only starting on a club race here or there then I suspect you will be able to make a Leopard last a lot longer than 20 hours (assuming you run it a bit rich and cool). Now if you are racing every month or try to get everything out of the engine each day at the track then that 20 hours will happen at 20 hours.