The way I remember it, the Space Marine bike and Ork bike are extremely similar in the seat, so your ork should fit on just fine (and let's face it, even if it doesn't it'll still look Orky). As for the Grand Master bike, you might have a bit more trouble. But, again, Orks are great in that you'll be able to force him to fit on it and even if it looks wrong, it'll look right.
The Eldar one would be interesting, because it's not the same as the SM or Ork bikes and it's made for the riders to sit on it like a crotch rocket. I'd see if you can find someone that has one without the rider glued on to test it out.
