It is not currently possible to define EventRegistrationFields or RegistrationTypes for an Event through the API either when creating a new event record or updating an existing one. This is a significant gap that severely limits the usefulness of these functions.
These fields are populated in the clone event function, so it should be relatively simple to implement the functionality into create & update.

This functionality is important to us (Charles River Wheelers Bike Club) because we have certain registration fields and/or ticket types that apply to some rides and not others.