that's just a 2 bbl mercarb, pretty easy to rebuild. You'll be in shock for what a new one cost. If you can find a 2bbl Rochester off of a early late 888 or 898 Mercruiser, it will do much better than that mercarb
if you got the money to spare do a upgrade and fit a 4 barrel manifold and carby---a kit is not that hard to do if you have some mech skills and a good memory were things go back---you can take pictures of each stage as you pull the carby appart to make sure it goes back the rite way -----