I've got both engines mentioned, CY is a 2-bolt and the zenoah has a 4-bolt head design, and to be honest in all the time I've had them I've not seen any difference in them I terms of reliability, and the performance difference as standard was fairly minimal. The CY has the noticeable grunt over the 26cc but then I had the latter ported and screwed around with it a little, it's now quicker than the 29cc, and it's also in an FG MB.
Either way, they're both good engines in my opinion, and for just bashing or club racing they'll be ideal. Perhaps your mates hasn't got it quite tuned up how it should, though as u say it is in a heavy truck, maybe geared differently too.