Here's that motor mount pic that Weeds asked for. It may look different than what you have, this is the Hen Long Work Boat. As you can see, a hotter Mabuchi can motor will fit nicely into the intended mount. HOWEVER! This image is possible because I had to open it up again.
(http://i163.photobucket.com/albums/t316/soccerdad_photos/004.jpg)
It seems two things happened:
1. The motor loosened from its mounting screws
2. The resulting friction against the mount experienced by universal heated up the motor shaft u-joint and the hole thing came loose from it.
It seems that with a stronger motor, the friction fit of the stock u-joint is inadequate for prolonged runing....or was the sudden reverse move by the Cub Scout at the controls the cause? Who knows, but I'm going to upgrade the u-joint to something proper.
And tighten those screws down with Loc-Tiite!