Is it the kit from BikeBerry? If so,my 29" build will get about 25 mph, but it is screamin. The kit comes with a 50t instead of a 52t (advertised), which actually helps with top end. The grips that come with the kit are not what is shown. The mounting plate holes do not match up with the bolt pattern on the bottom of the engine (will need a dremel to correct this) The throttle cable is too short, removing the nut adjuster on throttle linkage and bending linkage forward will allow the cable to work. The chain lasted about 50 miles, although most will say upgrade the chain to a #41 immediately anyways. The fuel tank is really small, most other kits at least come with a 1/2 gallon. Also if installing this kit on a multi-speed bike, the wide crank kit sprocket is only designed for a single speed (chain will not fit and this is same with all kits) although grinding the teeth will correct this. And of course, replace the bolts and nuts. There is probably more that I could add, but off the top of my head these are the concerns I had encountered with my recent build with this kit. All in all, for the money, what more could you expect? I think for the price really its not bad.
As for the gearbox? Most will say its junk. But the negative reviews are only on the older version of this gearbox as far as I can tell. I have not seen any negative reviews on this new gearbox. If the gearbox goes out at 1000 miles I will decide then if I shall get something different to replace it. The gearbox does come with a 6 month warranty. All in all, this kit allows for alot of fun for a little price. Treat it like a motorized bicycle and it will do you good.
Will I buy this kit again if I had to do it all over again? The answer would be yes. Although a different sprocket for a little more top end would also be in the mix. I want to cruise at 25 mph, not be there at WOT.
Hope this helps...