I come from the old school
of "if it ain't broke, don't fix it". Why would you remove a functioning engine that is too big to replace it with an engine that is tooooooo small. At the rate of one h.p. per 500 pounds, you should have an engine of 17.4 h.p. With the added cruising gear
and yourself of another several thousand pounds, a 22 h.p. engine would be in the proper range. The 12 h.p. is ~50 percent too small. Your 30 h.p. is ~25 percent too large. I would opt for an engine that is too large, especially when I am driving into a sea (wherever we want to go is to windward).
Keep the engine you have. It is always to keep the devil you know rather than take a chance with the devil you don't.
p.s. Fuel consumption
is not a major factor in either engine. The spec sheet you provided indicates that the boat has a 45 gallon fuel tank
. At 1/2 gallon per hour, you can do a lot of slogging to windward.