If you are looking for just a smooth finish for paint, you can use a clear acrylic (like future) clear coat, and then bondo/body putty.. And lots of elbow-grease, naturally.
I want to remember that Dragen is an International Racing Class, so the sizes and specifications are published, so that the size and dimension of the mast, boom, etc. is all called out somewhere. Also means you probably can find what material the hulls were made of, too (which could be diagonal planking, plywood, steel, or even aluminum--so wood grain or straked planking might not be at all to scale).
I'd for sure send a message to Billings (if they are still about) to see if they still have the plans available.
That mast is probably a rounded rectangle or oblated ellipse of some sort; I'd bet money it's tapered fore-and-aft, and likely athwartships too (this was, and is, much in fashion in racing rigs, where lean is mean).
When you start planning out the rigging on the mast, keep a strong idea on having it be removable, as that mast is likely to be 2 to 3 times the length of the hull you have (or 6" taller than any vehicle available to transport the finished work in--[expletive] Murphy).