Get Ben a little 2 Stroke Aprilia RS250 Chesterfield if possible and he'll keep up just fine. They've been know to hit around 160 mph on the track but only around 130 mph or so on the street. An Aprilia Ditek 50cc Scooter properly set up will get you just 65 to 70 mph but should keep up just fine too however the direct injection system on those 2 strokes can be a bit fussy especially if someone who didn't know what they were doing has been tinkering with it. Best!
Ive cleaned many moped tanks with works toilet bowl cleaner half bottle and fill with water let sit few hours leave cap off drain rinse out put vinegar to stop process then flush 2 cycle oil in coat tank