Those are some awesome engines, aren't they? Pricey, though.
You don't even need to spend the big bucks to see a significant performance improvement over stock from the Zenoah G26 engines. Simply adding a ported top-end, like this one:
http://www.davesmotors.com/store/product1829.html
gives a ton of bang for the buck.
I guess it all depends on what kind of power output you need for your target application.