I too had to reinsert the square drive shaft end on a U-33C my friend owns. It appeared impossible for the shaft to disengage from the square hole it is designed for but never the less it did just that. Like Dave Horn says it's a delicate but definitely do-able repair.
As with most repairs you'll need at least 3 hands to complete the job and in that regard it's advised to enlist a friend to be there to "hold that part steady while I ....etc.etc.etc". You may need to rotate the wheel sets during this process to allow the shaft to "square-up" with it's mating socket. You will need to remove the single screw that holds the truck to the chassis and the JST plug on top of the truck. Go slowly in all the repair and you'll do fine.
The repaired engine ran just fine and continues to do so. I believe the initial problem came from rough handling of the engine by the owner who has been known to lift engines up by the trucks for whatever reasoning on his part. I try to lift diesel engines by the lower edge of the shell where it engages the chassis and if possible always use 2 hands for added safety.
Mark
