According to Rule 23
Part (c)
(i)
A power-driven vessel of less than 12 meters in length may in lieu of the lights prescribed in paragraph (a) of this Rule exhibit an all-round white light and sidelights.
So the question becomes: Is your boat less than 12 meters in length.
The form for determining the length of your vessel is at
http://www.uscg.mil/hq/msc/T3/cg5397/cg5397.pdf
According to that form, you do not have to include include the parts of the boat which are not part of the bouyant hull envelope in the overall length. That means the pulpit on your boat would not be included in the length for Coast Guard Regulation purposes.
I would say that you do not require a masthead light since you are under 12 meters and have an all-around aft light.
http://www.navcen.uscg.gov/mwv/navrules/Rules/Rule23.htm