I have been having trouble getting the correct tooth count and offset sprockets for the large drum hubs I use. Finally I just had them made. These sprockets use an adapter plate that carries the drum brake bolt pattern and then I went with split sprockets to make gearing changes easier. The split sprockets use an standard bolt pattern used in Kart and Jr. Dragster racing.
Here are the hub components. Stock sprocket is shown to the left, adapter in the middle, and new split sprocket on the right.
Here is the hub assembled with the new adapter and sprocket. Also shown is the bicycle side freewheel sprocket.