Not bad at all. I had an idea to build all sorts of wacky vehicles like that for the kids. There's a trader on our equivalent of eBay who sells new 80cc motors for NZ$90 or about 30 quid, but the brakes are more of an issue. I think using MTB disk brakes might be the go as a lot of bikers ditch the cable operated ones for hydraulic.
The only issue I have with the cart in the video is the exposed chain drive - in several shots, you can see the rider's trousers or shirt sleeves are at most a couple of inches away from it. And I'd try to get a lower CoG by mounting the wheels wider to drop the main chassis down.
Looks like a laugh. I very nearly bought one o those road legal buggies to use as a runaround but chickened out at the last minute.
A friend loaned me one for a few months while he was away last year to try out. it got more attention than if it was some kind of exotic supercar every time I stopped at lights or parked. Great fun but v cold on the winter.
Anyone needs any help hydraulically i am now retired and have loads of free time on my hands and will gladly give free information. *MOD EDIT - HYDRAULIC BERTIE IS MANAGING DIRECTOR OF THE COMPANY IN THE LINK* Send me any queries you have glad to help you youngsters out there !!!