Technically, no, you should pay duty.
Practically, even if you're stopped (which is highly unlikely) when you explain that you bought some old vintage motorbike parts at a car boot sale for a few bucks, I doubt anything would be said.
But also on the practical side, you may be prevented from taking anything on as hand luggage if the security officer thinks it could be used as a weapon.
Personally, I'd put it in my checked luggage.