Hmmm, I'd probably risk it too. Not that I actually need that sort of thing, and would never use it, but.
Looking round the web, if it's less than 50cc, and capable of no more than 30mph, then it's classed as a moped. It would need an SVA, which it has zero chance of getting. Without that, can't get MOT tax or insurance.
If its bigger engine and faster, it's into Motor bicycle territory. Same thing, still needs an SVA to register it.
Only difference is the licence category required to ride it.
Now if you happened to take the chain off while on the highway, and put it back on for offroad use, or Zombie apocalypse, I can't see there would be an issue. The fact that the engine is attached to the frame, and not in a box on the back, shouldn't make much difference, if it's not functional.
Dunno if I'd want to explain that to a magistrate though.
Obviously, taking the chain off would mean you can't pedal it............