2016-10-01: part 2 updated to leave a little more gap to the gear wheel.
As the original gearbox started to break and disintegrate of the reely kaiman, I finally decided to sketch a new one. after a few trials this is the final result. make sure to start with the diameter check to ensure the bearings fit and the 550 motor fits smoothly into the hole, then take some time and print. you would need some M3x 8 screws to mount the two housing halves and M3 x 23/25 to mount the motor to the gearbox. I cut the threads not with final thread (old fashioned tools), so the screws are tight in the thread.
As long as the gear-wheels are in perfect condition this is an easy way to make the old toy working again. add some good grease when doing the final assembly. I use wheel bearing grease.
have fun