i got a 18 hp briggs given to me and i was thinking of upgrading my 11 hp honda gd to it only prob is it looks like a rotary engine with the plugs on the side of the block instead of a standard v-twin think it will work or does this sort of motor lack rpm or torque to push a 16 foot jon boat.

do you have any pictures of the motor to see

I've heard you get more rpm's from a rotary, but that they don't have much torque. Might just have to go a pitch down on the prop to compensate for it. I'd roll with it, definitely an upgrade to what you had.