Hi,
with 14" wheels it is unlikely you will even be able to take off in 1st with a Mini ratio in the diff so the easiest option, based on what you have said, would be to take the gearbox from your 998 and put it under the 1098.
You can count rotations of the driveshafts or if you remove the diff the ratio and part number is stamped on the crown wheel.
Terry