Graupner B 28 Speed Boat
A 550 would have more torque than my 540 and could drive a bigger prop. Also using a LiPo pack would save weight compared with my old boat.
Plenty of motor mount options - MFA do metal ones and there are vac-form and GRP mounts available too. I like the ones that cradle the motor but you may want to add watercooling which could complicate the mount.
Minispeed with standard 540 (Not my boat)