I'd prefer something a bit harder and more rot resistant - douglas fir, larch, cypress etc, but it should be a pragmatic decision - what can you find locally at a good price, perhaps a recycled spar, perhaps a tree straight out of the forest?
GRP doesn't make for a good unstayed mast - heavier and more flexible than its rivals. But if you can find a tapered tube that would make a topmast for an aluminium tube, it's a possibility.
CFRP makes a good, if expensive mast for lightweight boats, but is a little over the top for a heavy displacement carvel boat, I feel.