Yeah they share shoulder pad size. I think the trick to keeping them still looking like terminators would be to keep the backpiece. The vents and the boxy shape are pretty defining I'd say.
I'm not at home so I can't check my models right now, but are gravis legs thicker than the standard mkX ones? For bikes you could potentially use the Inceptor legs (especially in the new kit, the "car seat" one looks similar to riding a bike). Since they're definitely longer, skip the bottom part of the bike where the pedals are and then those silly shock absorbers on the Inceptor feet as your foot-on-pedals... Unless of course the gravis IS noticably thicker, then it may just look stupid... That said, a primaris on a regular sized bike might just look silly all together.
You could probably pop them on Twolves and they'd still look alright though.