uavwmn: I haven't made that Port yet, but I don't beleive they require any fortifying when you are completed. They are designed for 18 - 19 % ABV, as I understand.
Having made Ports, and if I were to fortify (for what ever reason), I would use everclear or something that is as tasteless. Unless you have a certain flavor that your going for. You can always try small measured mounts when you ready, for instance, 1/4 cup Port to 1/4 Tsp everclear or other fortifiers to see what you would like.