Ok, basic idea here, instead of getting Knightmare Frames, Lelouch and possibly the Black Knights get Kamen rider systems. Much smaller than the frames, but can more easily dodge attacks and have tons of power behind their blows (Not kidding, even the weakest of forms, Kuuga Growing and Den-O Plat, have had at least half a ton worth of maximum force behind their blows, more for finishing moves. And while I can't speak for growing, Plat Form never won a battle by itself, it always either transformed, or get smacked around until help arived)
Mobility isn't an issue, most Riders come with motorcycles as well. For balance sake, you can have either Brittania will start deploying some of the bad guys of the shows, there will be only a handful of suits on both sides, and/or some bad guys will start to spring up as third factions. Also, if you use this idea, don't feel restricted to actual riders. Stuff like Kamen Rider Gurren or Lancelot are possible.
(Said information on the force of attacks come from Kamen rider wiki's, which themselves state to get their info from japanese Kamen Rider supplimentary materials, as a result, only the main and occasionally secondary riders get stats)
Edit: for those curious, people tend to simplify Rider systems down to three types, although some tend to skirt the line.
Technological: One of the more popular types, these rider suits are usually transformed by technologically grounded belts, phones, mechanical animals, weapons, and such, or are enhanced by machines. Kamen Riders who have used the tech method include Kamen Rider Faiz, and the related Faiz gears, Kabuto and secondary Kamen Rider IXA. Kamen Riders from the Showa era, who were largely cyborgs, mostly fall into this catagory as well.
Mystical: The belt of these riders, and their powers, tends to be grounded more in fantasy than the mechanical riders, although this has been used in various ways. A large chunk of riders can be classified under mystical, from Kamen Rider Ryuuki's riders getting poer from the monster's they're contracted too, to OOO using a transformed sealing stone and medals to transform, to Kiva using the bite and power of a bat and other monsters. Late Showa/Early Heisei riders, like Black/RX (As far as I know), ZO and J are mystical riders.
Organic: By far the least used rider type, these riders tend to transform their bodies into their rider forms and usually, but not always, use their bodies as their only weapons. The most famouse riders of this type are Kamen Rider Amazon, who's origins tend to be mystical, Kamen Rider Shin and secondary Kamen Rider Gills, although the Hibiki Riders, who all transform into oni to become riders, could also count despite using weapons, as could some of Kiva's forms.
For the most part, both sides would use Tech Riders, with mystical and organic ones being better suited for those who would otherise, or still do, have codes or Geass.