If you are involved in an accident and it was proved your motor cycle was unroadworthy your insurance company may decide not to pay out. It is also possible that the police may then want to make criminal charges if neglegence is an option.
Making a claim will not automatically lose you all your no claims bonus. Most insurers will relegate you only two years meaning if you had 5 years no claims then you would still have 3 years. You can of course opt to protect this with an extra payment.
All motorbikes are classified into an insurance group. The higher the number category your motorcycle is in the more you will pay for insurance. Normally this relates to the power of the machine.
It's a common misconception that having fully comprehensive insurance means you can ride any motorbike. Unless your insurance states you are allowed to ride other motorcycles do not take the risk, no matter what the circunstances. If you are unsure and your documents are unclear, ring your insurance company to find out.
One way to save money on your motorcycle insurance, especially if you are a young rider, is to ride a cheaper motorbike with a smaller engine.