My wife and I went on a ride last year on our 50cc scoots. The round trip distance was about 35 miles. The group stopped halfway and this was a good chance for us to catch up. The only bad thing about the smaller scoots was that we were always at the back of the pack and when we weren't, we were getting passed by everyone else. Also, by the time we got back to the home station, we were at the back of the beer line.
Most of the group, about 100 or so scoots, were vintage. They kept an average speed of 50-55mph. The fastest we could go was 35-40. It was this event that caused us to purchase bigger scoots.
If you have a lot of 50cc scoots, you could have a separate ride just for them. If not, just let them know that the average speed of the group will be faster than they are.
Personally, we liked the idea of a slower ride. In fact, if it were offered both ways, I'd take the slower scenic ride as opposed to the white-knuckled one.