I'm impressed, 91 mile trip on a 50cc scooter so basically you averaged 22.5 mph with stops.
A bit sitting. First one and half hours goes well, that means no pain in the ass. Ten minute stop walking helps for next hour drive, lets say well enough not perfect. The last 45 km sometimes good, the feeling you are done with the trip soon, sometimes small only few minutes break to stand and walk are needed.
The body of Like 50 cc is same than 125 cc, I mean it is not the smallest moped, I think it helps. It runs around 50 kmh, rarely less when engine is heating a bit but runs and idles.
What also counts is motor oil, the last time I changed oil just before long run, and these days never over 1 000 km with same oil.
Summarum: 50 cc carries 120 kg good, if you are able to sit and try