If you are looking to frame mount for your GXH50, try here:
Bicycle Engines, Motorized Bicycles, Bicycle Motors, Motored Bikes: 2-Stroke & 4-Stroke Kits!
I don't know if they list the kit separately on that site, but I have heard of people buying it there.
Same about this place:
ATV,Dirt Bikes,Mopeds,Mini Bikes,Electric Bikes,Motorized Bicycles,Bicycle Motors
Just a piece of unsolicited advise- for the frame mount and gearbox- stick with the Grubee Skyhawk II. There is another kit out there, but I have heard nothing but trouble. You can tell them apart at a casual glance- the Grubee gearbox is a symetrical, kind of oval shape. The other one looks like a number "8" on its side. Also, the Grubee has an adjustable mounting plate that fits more bikes. The other one uses a non-adjustable plate.