The stock 50s that come on Rucks (called a GET motor), are only good to about 65mph, and that's with an insane amount of modifications to them which just isn't worth it.
The most common engine swap is the GY6, a 150cc that can do 65mph in stock form, it's just a pain in the ass to wire up, but it's worth it.
Combine a GY6 with a big bore kit, and depending on the size, I've seen them run from 75-85 with a 160 kit, to over 90 with a 180 kit. All of those have trans mods as well: racing clutch/springs heavy duty variator, heavier roller weights in the variator, etc...
He pulls the front up due to the stretch, but I'd think that an unstretched GY6 Ruck with 2000RPM clutch springs would be a wheelie machine :lol: