Use the agk gear ratio calculator, you need something like ~ 12.*** something overall reduction...sounds like your geared a little tall. Might want to consider using something like a 56t sprocket rear sprocket.
Also I'm not sure if they make sprocket adapters for the multi-geared hubs, there usually made for coaster brakes hubs.
Plus any good sprocket adapter and sprocket together is easily going to cost 70+ after shipping.
Another route is finding a decent rear disc mountain bike rim and buying a sprocket from agk of your choice that mount to the disk brake hubs directly without any special adapters