with today's technology & advances in metallurgy even with a 47mm or 50mm stroker crank you can make it rev to 19,000+rpm like the go karts do, the only problem is that the basic scooter carter is of a compromised design where packaging more than performance is the goal but with some ingenuity, dexterity with a grinder & some epoxy it can be fixed.
hp is also dependent upon torque output at high rpms, a linear torque curve that extends at overrev will net you more hp, for sprint racing especially in shorter distances torque is much more important in accelerating from a stand still, sure you may give up some rpms with a stroker crank but you can make it up by tuning the vario & changing gear ratios.
in dragracing peak hp isn't very important, what's important is to have a fat, linear powerband that extends from 9,000rpm all the way up to 15,000rpm, most of the time dragraces are won or lost at the starting line so you need torque to pull you out from the starting line as quick as possible, over here acceleration up to 50m is just as important as top end power, the quickest 70cc, 90cc & 125cc dragscooters over here have only a difference of a few kmh in topspeed but the biggest factor is the acceleration in 0-50m & the 125's accelerate like a fighter plane hooked on a steam catapult on an aircraft carrier, that acceleration you can only get with a lot of torque.
look at where the trend is going, most tuners are no longer satisfied with just 70cc they want more bang for their buck, the 2F4R 78cc & FHT 84cc are filling up that niche, way before the only course was to convert cilinders from 85cc dirtbikes & fabricate a custom crank to fit into the scooter carter, if the demand goes up maybe manufacturers will start making race kits with much bigger displacements, at the moment the FHT 84cc is starting to look pretty good, with a custom pipe it should make around 28hp to 29hp with gobs of torque in the midrange, maybe after we complete the 2F4R project we'll get us a set of FHT 84cc race kit to build another dragscooter.