There aren't many moped or scooter riders on fuelly, which makes this a small hiccup. Maybe add generic Make selections of "Moped" and "Scooter" then let the user define the specific make and model within the notes. When/if there are enough users in the range you could use the existing data to create a starting point for the make/model selections until that happened it would be fine just having somewhere to track them from.
I wanted to post this in the public forum to see if anyone had a better solution, to suggest.
Have you seen the Browse All Motorcycles page? Scooters and Mopeds are in there too, and you can browse by make. For example, you can see all Genuine Scooters. We don't want to move away from actual makes to something more generic and lose that ability to browse by manufacturer.