S.H. Figuarts stand 13-14cm, I think? That's probably smaller than 1/12 scale; definitely shorter than most Revoltech or Figma figures (which themselves aren't to scale). In other words, any bikes for the S.H. Figuarts line will have to be specifically made for that line of figures.
I could recomment checking out some of the previously produced bikes for the series, but if I remember right, the older Shochaku Henshin figures (NOT the Figuarts ones, the older version) had varying sizes (something about Blade figures being smaller than other series figures?), so there's a caveat.
Also, I do remember that the Kabuto Extender seemed to look really runty when you put the Cast-Off Kabuto figure on it (it looked like a mini-scooter).
AFAIK, S.I.C. figures are 'true' 1/12 Kamen Rider figures in terms of size and build. 3.75"-4" figures are around the 1/18 scale (Star Wars, G.I.Joe, some specialty military figures), and finding motorbikes for either should be no problem--model shops (especially diecast collector shops) stock 1/12 and 1/18 scale bikes, and I think some people would custom said bikes to match.