I suppose the polite way of putting it is these engine's need a bit more development.
What I've read is that they've the same power as a tuned 26-29cc Zenoah (4-5hp), but have a lot more torque.
As mentioned, to make the most of this, you gear higher.
Don't know about the clutch - I reckon they rev quite a bit lower, so definitely DON'T consider a 8.5k (red spring) clutch, if it can be fitted.
I liked the idea when I first saw these a few months ago on this forum, but I'm not convinced - very expensive for whatever gain there is ... it is nice to say 'mine's a 62cc', not a 26cc !!