I think you mean 154FMI rather than 154FHI. The engine is made by Zongshen and it's one of those with the semi-automatic clutch. Forget the make of the bike, they're just assemblers of other people's bits. Look for a source of Zongshen spares. Specifically in dealers and websites that sell and service monkeybikes or pitbikes.
Is this anything like it?:
