Making a direct mold of an existing top (a positive or male) is a very simple operation, requiring a release agent sprayed onto the original, then a thin gel coat for smoothness, then a lay-up of cloth and resin to an appropriate thickness (I would go for 3/8"+ to make sure it holds a true shape). Were it not for the abrupt "creases" made by the top bows, a convertible top would work pretty well, or could serve as the basis for a male mold to be modified with Bondo until it has the shape you want, but that takes a lot of time - might be easier to copy a Miata top or something else that's close and then modify THAT to fit a Speedster.
The male would have to be reinforced with plywood glassed on the outside - not a big deal and done while it's still on the positive to guarantee its' shape. This is also the time to cut in the rear window hole, and the piece cut out is the shaped mold for the rear window plexiglass, since it'll hold its' shape to a higher temp than plexiglass will, allowing the window to be shaped in a small oven under low heat.
Then, making a reverse of that (a negative or female) in fiberglass, not a metal buck, would be an equally simple operation. That mold would then have to be reinforced with glassed-in wood on the underside to hold it's true shape. Once the wood reinforcements are in, it's good to go. It could then be "finished" with more fiberglass or body filler for accents or trim points, and worked until it looks like a mirror of the original, or is modified from the original to a desired shape.
I like Alan's idea of grafting on a convertible tops' header bow, but it would be just as easy to graft one onto the male, and then crank out a female mold capable of making copies with the bow fully integrated with the rest of the top. You could easily test out (for looks) the male as a real top on a car before you reinforce it or mold a negative from it. You just have to take into consideration the difference in final dimensions. This positive is also where you decide how to handle the differences in rear cowl curves and design in a rubber gasketing system (a tubular, rubber gasket like on early Corvette tops sounds good).
All this could be done, full time, in a couple of weeks on the outside, leaving you with a fiberglass mold ready to make pre-production copies.
THIS is how it's done in the small-volume boat building businesses in Rhode Island (and in a lot of the larger volume places around here as well). ALL of the smaller boat builders around here are using fiberglass molds for boats of 7 ft. up into the 20+ foot range, and they last for years and hundreds of copies. "Tooling" involves having someone trim or tweak the mold til you get it right.