The Go Kart Galaxy website doesn't seem to list any hubs that fit that sprockets (6 on 3.146") bolt circle patern on their site...
Have you already tried sending them an email asking if they have the hub you need, but just don't list it on their site?
Here's their email:
sales@gokartgalaxy.com
Go Kart Galaxy buys most all of their sprocket componets from a manufacturer called ASUSA.
The ASUSA website doesn't seem to be listing the any hubs with the correct bolt circle patern you need for that sprocket either:
https://azusaparts.com/product-category/hubs/sprocket-hubs/
I checked out your profile pic, looks like your building a shifter racekart, right?
I can probably help you find a sprocket and/or hub that's specifically made for shifter racekarts like yours, If gokart galaxy doesn't have the hub you need; but I'll need more info to help you:
1-What brand & model of motorcycle engine do you have on your kart?
2-How many teeth does your engines 520-pitch counter sprocket have?
3-How many teeth are on that Gokart Galaxy axle sprocket you already bought?
4-Also, what's the diameter of your karts rear axle? Is it a 30mm, 40mm, 50mm or 1-1/4" diameter?