My investigation leads me to this sprocket in their website -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
thank you magnaassuming the riverside is the same is the clairmont, my steerer tube is 1".