You need a small pump to pull the fuel out. They sell small piston pumps at the hardware store that can do this.
Cutting the tank apart is probably the only way to get the tank out, since it is very likely that the tank was installed before the hull and deck were joined.
The pieces of the cut up tank shouldn't be all that heavy, so you shouldn't need a crane or hoist to pull them out.
As to how large a tank to put in... put in at least a 12 gallon tank. That gives you enough fuel capacity if you should decide to take a longer trip, and is large enough that you don't have to continually refill the tank.
Since these tanks are mounted in the lazarette, I am hoping that you're planning on using a deck fill to fill them rather than trying to muck around with pouring gas into a tank in the lazarette itself.
I like the idea of having two tanks and think it is a good one, but it will require a bit more work since, you'll need to add a second fuel deck fill and vent line, as well as a diverter valve for the fuel tanks.
Modern plastic tanks are perfectly serviceable and will last a fairly long time provided the tank is securely mounted, well supported, and protected from chafe and UV.
Cutting the tank out through the hull strikes me as good for the boat yard, bad for you and the structural integrity of the boat. Even glassing the plug back in from both sides would probably leave the boat weaker than it was before doing so, since the strength of fiberglass depends on long continuous fibers...and cutting a hole in the hull destroys their continuity.
As for actually cutting the tank apart.. Hmm... That's always a problem, especially making sure that all of the gasoline is out. If you're going to be cutting the tank apart in the end, cutting a large inspection port into the top of it could be done using hand tools and allow you to make sure the tank is empty and vent any remaining gasoline vapors out of the tank.
Once the tank is empty, cutting it apart with a sawzall would be relatively easy. You'd proably want to cut the most accessible parts of the tank up and then pull the rest in one large piece if possible. You'll need to remove whatever fasteners are holding the tank to the hull. I'm guessing that it is bolted or screwed to the hull in some way.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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