I'd go for a 36mm motor and a 120 amp esc, that motor isn't to efficient, you'll be real close to the limit of that esc.
Check out the Tacon motors at Hobbypartz.com.
Most people are having good luck with the swordfish and turnigy 120 amp esc.
Soon as the water turns liquid again I have a swordfish 120 I'll be trying in a 1/10 hydro.
I'm not familiar with your boat but a 36mm motor should have the same bolt spacing as your 550 does. And be about the same diameter.