What technical modification to a motorbike will generally void the issued operating licence?

You answered correctly. Changing the gear ratio of the pinion and chain wheel or altering the engine power are technical modifications that can affect the safety, performance, and emissions of a motorbike. Such changes require a new inspection and approval; otherwise, they void the operating licence. Installing a tankbag is not a technical change affecting the licence. Always make sure that any technical modification to core components like the engine or drivetrain complies with official approval to keep your vehicle legally on the road.