Most of the kits are the same basic kits from China.
It seems like some dealers get kits from different factories, which will produce different quality.
One dealer with a high-quality reputation motor is Chris Hill. he actually works on the motors himself before selling them I guess. A little more spendy than the rest. CH80 Bicycle Engine Kits by Chris Hill, Motorized Bicycles, Bike Engine, Bike Motor, Bicycle Motor kit
I don't have any first hand experience with him though.
Other than that, I think it probably comes more to service than anything with the China motors.
I know Easy Rider is using a Morini (i thinK) motor. Something like 6hp on a 50cc motor. Might be something to look into if you've got the cash.
Welcome to the forum.