Depends what bits are missing. For struts there are companies out there that produce long lengths of aerofoil-section plastic that can be cut to length and then have the ends shaped. pretty much everything else can be made out of plasticard or stretched sprue. You might also ask on the modelling sites in case someone out there has spare parts you can use.
If you can be more specific about which parts you need to replicate, I'm sure people on this site will be full of ideas for how to make new bits 'n pieces.