The 50cc Sento is made in China and uses the Chinese QMB139 standard engine, sometimes called a "GY6". The larger Sentos/New Sentos use a Kymco-Taiwan 100cc/110cc engine, mostly similar to the Kawasaki/Polaris KFX90 ATV. The confusion comes from the fact that the two Sentos look almost exactly the same. Upon close inspection, the Taiwan 100cc engine is much "fatter" then the Chinese engine because of the hefty 100cc variator and clutch.
The low-tech standard Chinese QMB engine is much cheaper to buy and service. They're available everywhere on eBay, etc. Parts are incredibly cheap and plentiful. The 100cc Taiwan engine is a jewel, a fine piece of engineering, high-revving and fast - but worse than a Ferrari for parts availability! Any information on parts you will have to find from local forums like this. Kymco won't tell you anything! Bring your questions here, and somebody will try to point you in the right direction.
PS: Sorry about the poor images, but this site is not really friendly for images! If you click on the thumbnail below, you should see a better image. Disregard the error messages at the bottom of the page. Hopefully the site may be debugged one day!
