You are not thinking like a cheater. Of course, it's unlikely to defeat the x-ray itself. So you do what any cheater does: find holes in the process.
For example, it's unlikely that all bikes could be x-rayed at the same time, in the seconds just before the race. The process takes time. That means that bikes will be sitting somewhere--theoretically in a secure area--for some period of time after the x-ray. This leaves open an opportunity to bribe someone, or breach security in some other way, to make the desired modifications after inspection.
Another possibility is to find a way to swap out a look-alike cheater model, for the duly inspected version. In any long bike race, it's unlikely that every foot of the course is under the watchful eye of officials, leaving room for a secret exchange. Or even if every foot is being watched, there's still the possibility of paying someone off to look the other way.
So yes, your x-ray itself would be hard to defeat. But that does not make the method foolproof.