Howdy,
First ... are your building the lug or sprit rig? The Lug rig will have little problem with your approach, but the sprit will be a bit more tricky.
The below applies to the sprit mast but might indicate some of the things you will have to think about with the lug. The taper is not so critical with the lug as it mostly provides an aesthetic function- which is still important as lug rigs look clumsy with untapered masts.
It would make the sprit mast very stiff and the boat harder to control in stronger winds and make the boat a little harder to right from capsize.
Have you had a look in your local phone book to see if you can find a lumber yard, cabinet maker, joiner, boatbuilder in your area and ring for a price to do the thicknessing. They usually don't charge very much and then you would have the perfect mast.
Also two 25mm pieces add up to 50mm. This means the top part of the mast will be solid and the narrower side pieces will go down to nothing.
So this means there will be some construction trickiness.
If you cannot find anyone to take the thickness down you could build a hybrid mast.
On the NEWS section for the
PDRacer website there is information about building a solid mast for the PDRacer. It is calculated to have the same bend as the original mast. You could use narrow faces to make the bottom third of the mast have the same size as the outside dimensions of the original sprit mast as you plan. But when it became solid you would have to plane it down to match the solid mast drawing
Best wishes
Michael
PDRacer Mast 
