According to the E/F28 Spares List, only one transmission input sprocket of 31 tooth was offered.
A range of transmission output sprockets was listed:
14 tooth, (Standard Combination)
15 tooth
16 tooth
17 tooth
18 tooth, (Standard Solo)
19 tooth
Clutch sprocket was 18 tooth, and rear wheel sprocket is 51 tooth.
No idea where you would get one ready made. Best bet would be to find a good center hub and have a new rim put on it with your desired tooth count.
-Doug