56 rear sprocket for 428 chain.
https://www.amazon.com/JT-Sprockets-.../dp/B000GU1K8K
Combine that with a 13 tooth front JTF259 sprocket and your top speed will literally be about 45mph.
https://www.amazon.com/JT-Sprockets-...EKX1KQAAD&th=1
You are probably going to need to add a few links to the chain, obviously.
If it were me, I would probably start with the 56 rear with the 15 front though. You can always drop or add a tooth or two up front as needed.