Which kind of plastic material would be suitable for a clutch cover on a motorcycle transition?
Most plastics consist of linear molecules with little inter-molecular interactions, so most plastics melts too easy. The only ones you mentioned having a chance are polyuretanes and polyamides. They have (CO)(NH) fragments, that forms strong intermolecular hydrogen bonds. So, they are ones of the strongest polymeric materials 

And I will recommend aluminum anyway as it is much harder to screw things with aluminum.