Pit bike engine brands are no many. It should be noted firstly that the frame attachment points are all the same, it is very easy to change one engine for another.
The Lifan brand has a little bit down the performance range, but continues to deliver some very good engine 125. Take care there are a lot of fake sold under Lifan brand name.
The Japanese Daytona manufactures its engine in China from ZongShen factory. There is no warranty from them, there are alone to do no warranty and there are not the most reliable !
UPower is a french engine engineering office working to upgrade the engines since 2000. They prooved on the races since a lot of time.
Starting from 2014 some of their engines will be made directly from a full new manufacture under the brand name TOKAWA. There are for not the most reliable and the most powerfull.