Hi KnimRod,

On 7 GP3300 , direct drive, the Graupner 600 Race is a good replacement.
This motor has more torque compared to the 540 and is better suited than a car motor.

Watercool the motor and check for overheating on the first run, with two minutes intervals, don't run the complete pack, make sure to check!
As the Atlantio has a submerged prop, you may want to start with a smaller one, or one with a lower pitch, if the motor gets too hot.

Keep us posted.

Regards, Jan.