theres an IG dirt bike type conversion tutorial on the UK games workshop site, their design is abit expensive to make in my opinion, however it'd be worth looking over for inspiration.. i'll try and find the link for you..
You could try a space marine bike, now that on its own is too wide, maybe cut a strip out of the middle so the tyres and handles bars etc it won't be as wide. It may not look fantastic but it may be cheaper than building your own from scratch.
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