There is a website in the states that specializes in painting/restoring VMX plastic tanks. The thing is i dont know what its called. Ive posted the link on here somewhere, but dont know where so you just have to search. All i know is that the front page was red and they had a spray room full or plastic KX 450 tanks in bright green. The thing is, your likely to pay to $600 for a highly specialised job like this and its most likey doomed to fail even if you treat it with the most amount of care possible eg have a liner in it and drain the fuel after each ride and dont leave it in over night. And then what, most people will jump on these forums and winge and complain their $600 tank paint job failed. The plastic is oily polyethelene which was never intended to be painted. You might have a bit of success painting a brand new tank, but once its had 2stroke pre-mix soak through the pores in the plastic for 25 years your fighting an uphill battle trying to get permanant adhesian, even if you rough up the plastic first, use plastic primers and fuel resistant 2pack paint etc. In my opinion, your better off sanding and polishing (for people not requiring a change of colour), looking for another tank or buying a replica Clarke? tank or a NOS tank, or fitting a vinyl or plastic/fiberglass/carbon cover. DC plastics are now making plastic tank skins.
Another thought, for a few hundred you could get Clarke Tanks to custom make you a tank in any colour and it would probably be cheaper than getting a specialised paint job thats not going to last.
Ive seen you source some incredibly rare parts lately and i dont think sourcing a white Mugen tank is beyond your capability
