Firstly, note that the type of motor you're looking for is the "long type".
Secondly, the truth is that pretty much any motor with a pretty logo and a fancy name is going to be fairly badass in a King Arms gearbox that has been nicely shimmed and well-greased. Relative to your stock KA motor, they'll all be fairly "upgrade-ish". If you're wondering about your gearbox, I went into a KA M4 gearbox twice over the last couple weeks on behalf of a friend (the stock o-ring is a little anemic) and I was pretty impressed by what I saw: A nice lightweight piston, excellent shimming out of the box (no gear whine at all, very little play, no heat problems), good greasing, etc. That said, the rate of fire on the stock motor isn't super high with a 7.4V LiPo, and some workdesk testing confirmed that the motor doesn't have rare-earth magnets (i.e. neodymium, or "neo" if you are Googling/researching). If it gives you any comfort, you haven't lost an exceptional motor or anything like that
So there are a lot of interesting motors on the market that have various uses depending on what you want to do and what your budget is -- you have a lot of choices. Here are some I have toyed with recently:
G&P M120. It's a high speed motor and not a neo. I have found that even high speed motors that don't have rare-earth magnets are able to hammer out high rates of fire on strong springs (eg around the 400fps range) as long as you give them enough juice, use the right low-resistance connectors (i.e. Deans), and have good shimming/etc as mentioned above. This motor is an example of this. It's recommended by a lot of people and not super expensive either. I have a couple of these motors in both long and short formats and they're pretty good. This is a very safe choice.
G&P M160. High speed, high torque, and has some kind of neo or neo-like magnet (i.e. a lot stronger of a magnet than any stock motor magnet I have and stronger than the G&P M120). This is the stock motor in my G&P Canadian Carbine. It also sounds very nice when everything is properly tuned. More thwap, less whine.
Guarder Infinite Torque-Up motor. Similar to the M160 motor, but has a much stronger motor from my workdesk testing (seriously, keep this motor away from all steel items in your workspace
). It has a heat-treated (black) pinion gear. The G&P pinions use somewhat softer metals. If your shimming and pinion-bevel alignment is good, this won't be an issue. I am using this in my DBoys PDW and am quite happy with it. It is a little pricey though.
JG Blue motor (also sold as the Echo-1 High Torque Motor). Several US-based airsoft forums (airsoft mechanics, airsoft retreat, etc) have come to the consensus that the JG Blue is pretty much the highest torque motor on the market due to the very high quality neodymium magnets and a very high number of winds. The caveat of course is that your rate of fire will be fairly low, so if you want a responsive trigger with this thing you'll require high speed gears. This one is in my teammate's DMR, effortlessly pushing a spring that fires 450 - 460fps. This is usually the least expensive motor out of the batch I've mentioned here, but may be inappropriate for you if your gear ratio is stock and you wish for a decent rate of fire.
Personally, I suggest the G&P M120 long-type. Note that swapping the motor will REQUIRE
you to ensure that your bevel-to-pinion gear alignment and shimming is correctly re-done for the motor's unique height (each one is slightly different!) or you could get very bad (noisy and damaging) results. If in doubt consult a gun doctor.
Hope that helps