A sport 40 won't need nearly as much torque as my scale boat. If price is an issue, you might consider an S3305. It's specs are:

• Wide usage in cars and boats that require standard size, high torque servos
• Two ball bearings
• 3-pole motor
• Standard Futaba J style connector
• One year warranty
• Dimensions: 1.6 x 0.8 x 1.5" (1-9/16 x 16/16 x 1-1/2") (40 x 20 x 38mm)
• Weight: 1.7oz (1-11/16oz) (47g)
• Speed: 0.25 sec/60° @ 4.8V 0.20 sec/60° @ 6V
• Torque: 124 oz-in (8.9 kg-cm) at 6V 99 oz-in (7.1 kg-cm) at 4.8V

While it's got less torque, it's also only $41