Check the gearbox oil level: chances are the box is running dry.
This will either remove the problem or put off the day of disaster for a while.
Check for leaks from the oil seals when you've re-filled: they're easy enough to do.
As to interchangeability, a Cinq Sporting box will fit with very minor swap overs. Don't even think about fitting the box from another 1100 (non Sporting) Sei: it's far more hastle than it's worth.
They're different boxes on the 1108 SX and S to the Sporting one. They'll fit the engine fine, but you need to use the S or SX gearbox cables, probably modify the starter motor and use S or SX driveshafts. And you'll end up with a box with wet inner pot joints.
I don't know if this is true of them all, but was my experience fitting an 1108 S with a Sei Sporting box (which is a better way to go, I think, as the Sporting seems to have a tougher box, but still a real PITA).