Hi Dave,
The only condition behind me telling you this is that if it works out I can come for a ride
Have you tried Aero-Classics Heat Transfer Products in La Verne, California? They do alloy P-40 oil coolers and the heat transfer is slightly better than the brass/copper, they also weigh less which is a nice bonus.
The address and number is;
1677 Curtiss Court
La Verne, CA 91750
909-596-1630
909-596-1753 Fax
Im not sure if they do the 3-hole rotary-type vernatherm setup but as they are custom made and machined it may be possible without to much trouble, worth asking for sure.
Im sure you have tried Replicore in New Zealand as well, potentially that is what you have I guess.