I doubt a 550 would fit without more mods than it's worth. Besides that, it doesn't have digital proportional steering and most likely has a live axle meaning both rear wheels turn at the same speed when turning. This usually results in Under-steer or severe over-steer. The truck probably has a 380 size motor which is a common size. The problem you might run into is choosing between torque and speed. Higher turn motors have more torque to get you through rough grass but it won’t do much for speed. Low-turn motors are fast but week. Your best bet -if you want to put in the time and cash- is to get a higher powered motor of the same size. The stock pinion may be tricky to remove as it’s not designed to be but I’ve read that gently clamping the pinion in a vice and tapping a thin nail into the pinion hole should free it up. You’ll want to make sure you get a motor with the same size pinion shaft unless you want to burr it out.
After all that, though, It you might just be better off upgrading the batteries. Yours probably runs on a few AAs. You might be able to gut the battery boy and fit a rechargeable 9.6 in there. The extra power shouldn’t hurt the electronics and will give the steering servo more power to turn better.
Good luck!