Duane from Dax is a good guy and responds quickly to any issues that you might have but his 4-stroke is out of stock for a couple months or more. My first happy time came from Livefastmotors and they were very helpful and always answered all of my questions. If you are considering a 4 stroke rack mount, you may want to consider Staton as well. The Staton setup costs a bit more but the gearbox is guaranteed for life and all of the components are heavy duty and made here in the US. Also they have parts readily available if you ever need them. I suspect that the US-made Dax Titan or a Staton would be of higher quality than the livefastmotors offering though as livefast's are Chinese. I have had nothing but problems with my China-made bike setup. As far as the companies are concerned though, All three have good reputations. Good luck!
Duanes kit has a jackshaft and his engine runs counter clockwise but how the engine sits to me is more natural because the intake is in front,exhaust is in back,rope starter you can pull with your left arm while holding the throttle with your right hand while pedaling even.
I can't talk about the other kits because I don't own them but they look good too.
IMO, I think they are all the Same. Just like shoes your just paying for the name and not the quality, Yeah a dealer can argue their difference but all the same material. I'd go with the one that's cheapest and gets to your house the fastest. They all break , but which one will break first??. It's a Gamble, so roll the dice and go for it. Good Luck. Just make sure of your STATE LAWS before you take the chance and spend the money, Because i know it can bite you in the arse in the long run.