Is your motor threaded for a freewheel or splined for a cassette? People use the terms interchangeably, but cassettes and freewheels are not interchangeable.
In either case, it's an easy job. You can get 15t to 22t single freewheels, or you can use a cassette sprocket from 13t on up. The cassette sprocket offers you more control over chainline.
This is to express my gratitude to Justin of Grin Technologies for his extraordinary measures to save this forum for the benefit of all.