The replacement size for the 550 motor is 36mm brushless motors.
I have Hobbyking motors and Esc and had no problems.
ESC's I use the car ones (60 or 100 Amp) because of their size, weight and they run cool at all speeds.
Also they handle 3 and 4S batteries.
Batteries the 3 and 4S are about the same size up to 3700mah and the weight about the same a 3300mah NiCad(340grams).
High C on the batteries the better(40 or 60C).
I use the XT-60 connector ones, less change of shorting out batteries.
Motors D3548/4 1100KV 910watts 3-5S battery(3S=12210rpm,4S=16280rpm) or 3648-1450KV 1600watts 3-6S battery(3S=16095rpm,4S=21460rpm).
About 750watts=1HP.
I am using a 40mm x 1.4 pitch prop(code377000009) but I plan to order a 40mm x 1.6 pitch(code377000012),42 x 1.4 pitch(code HK-5B12x42) these are all 4mm shaft props.
The brushless motors have 3 wires to the ESC, I connect the centre one together and if the motor runs the wrong way cross over the other 2 wires to the ESC.
ESC setup using the program card 10% start power, advance timing on 4 (highest) and reverse force on 25%.
Give you nice slow speed, good reverse speed and good control on the throttle.