And leon was sitting next to Elvis in the spaceship, which I thought was
kinda strange, but then they turned to me and said:

The gear teeth in those boxes are well hardened, and should wear well
unless oil changes have been badly neglected. Examine the teeth for
obvious chips, pitting, wear ridges and scoring. If the faces of the teeth
are nicely smooth and shiny, they're fit for re-use.

The main points of wear are the bearings and the baulk rings. It's usual
to replace these components as a matter of course when reconditioning a
box.

You can get high-tech bronze alloy baulk rings from Minispares nowadays,
which are supposed to be a lot harder wearing that the standard sintered
steel parts.

You can swap the gears as an entire set, but don't mix-and-match between
them. (with the exception that you can swap the diff as long as you use
the pinion that goes with it).

I would use the gt cogs (the complete setup).
GT boxes are 'close ratio' (just a longer 1st gear on gt's).  Use the 3.44FD
too.
If you are lucky it will be an A+ c/r box.
You can tell if yours is an A+ c/r gearset as the part no on the laygear is
DAM3618 i believe.  It is stamped on it.
These are relatively much rarer that the earlier non-A+ c/r gearset due to
the fact they were in production for a very short time.
They are very strong, if you recon the box it will take whatever the
(tuned?) 998 will ever throw at it.
As AD has already said though, it is the bearings and baulk rings that often
require checking & replacement when reconning a mini gearbox.

best of luck

chris