You can buy sprockets that are all 1 piece, hub included. Surpluscenter.com has them pretty cheap, thats what im running on my shifter, pushing 40hp and i have had no problems.
I have never even heard of 428 chain. Why do you think that would be better for my cart? The chain doesn't seem to be a problem. And my current sprocket seems good.
Strength and durability, you guys tend to push things hard and for your powerplant it seems a better fit. I typically don't suggest 428 but you seem to break things.