I have a BMW System 4 helmet, about 5 years old. Never dropped and with a recently replaced visor, it is in perfect condition - except!. A plastic revolving part on the right hand side has broken and the helmet is therefore unusable - one side of the chin section keeps detaching. Can anyone tell me if replacement parts are available? If so, where? I'm planning to buy a Vemar Jiano Bluetooth flip helmet but would like to fix the System 4 for my sometimes pillion. Thanks in advance.