Hi Rob. You have probably already tried these options, but I would make enquiries on the MXTRAX UK forum vintage section, or the French forum called, Le Guide Vert. The MXTRAX forum seems to be dying off a bit lately, but the French site appears to be powering along ( although you need to be able to understand French, or use a google translator ). I wouldn't expect too many leads from the OZVMX forum, because a lot of the people that race or are interested in vintage motocross bikes, have moved on, because of a few that mainly post negative stuff or boring endless threads about various interpretations of rules.
Another place to look for OW parts, might be a guy in Italy, with a Facebook page called : Mastercross VMX Works Riders , Bikes and Parts . He often posts up pictures of interesting works bike parts that he has for sale. You can just Google the name to come up with his Facebook page. Good luck with your search.