here are a couple links to what you'll need
servo
SW-1210SG "Tall" Waterproof Aluminum Case Digital Steel Gear Servo by Savox
servo extension lead
Heavy Duty 15cm (6") Servo Extension Lead (Male/Female) by ProTek RC
aluminum servo horn
Hot Racing TRXF25SH06 Aluminum Servo Horn 25t TRX-4 (Stock)
bec
10 Amp Adjustable BEC by Castle Creations
as for the servo i linked above, it's just a reccomendation. i would go no lower than 250 oz/in @6volts. The bec will need to be soldered to the battery connection and the thin wires will plug into one of the receivers unused ch.1 ports ***do not plug into any of the telemetry ports. that bec i linked to is not water proof but all everyone does is squirt silicone into each end of the shrink wrap. they do offer a waterproof one but its double the money. the trx4 oem servo horn has a 22mm c2c measurement, the one i linked to is also a 22mm c2c and the spline is a 25t which will work great on savox servos. most servos do not come with long wires so you'll need an extension in order to plug it into a receiver . I would also get the bec programed to 6 volts by your local hobby store. i think they come preset 5.1v ?
some servos are a direct fit some are not i know my savox i took a hobby file to one side of the mounting holes and made then straight due to the weird mounting locating pins on the trx4 mount. also what ever servo you get make sure the servo horn spline count matches the servo spline count.
one more thing after upgrading to a better and stronger servo, reset your end points so you do not damage the servo or start breaking steering components.