I've also got a 96 Speedster and I did convert it over to a large 12v marine battery. But it was heavy and unwieldy to get in and out of the boat when I needed to charge it (my boat is in a storage unit with no electricity). I finally got a small 12v Lithium battery. It's been working great and has plenty of Cold Cranking Amps to turn over the engines fast... which the 2 small original batteries did not.
My only concern sometimes is, what if my battery dies for some reason? Well, flag down another boater or break out the paddle. At least with the 2 battery system, if one dies, you have the other to run one engine. If you're going to stick with the 2 batteries, just make sure they've got high CCAs.
Because of this, I may go back to the 2 battery system... plus the Lithiums are really light, small and easy to work with.