Hi and welcome to the Cruisers Forum.
I did a quick search on your engine/gearbox and found this thread
, which pretty well sums up your problem.
You'll need to verify the information in the above link, but if it is correct you could find it very difficult to fix, rebuild
or even to replace your gearbox.
You don't say what sort of boat you have, but with the heavier yachts that many members on this Forum have some of the smaller marine
gearboxes have not given good service
when operated continuously.
I don't have any great deal of gearbox experience. I specified a gearbox that had three times the required specification for my boat and have, so far, experienced no major problems.
I seem to recall
a rumour that the smaller Twin Disk gearboxes
are possibly a little more study as a replacement.
The procedure in the first link of sending the gearbox off to have the adapter plate accurately measured so it can be fitted to a new gearbox may be necessary.
I did not find any reference to the Kanzaki
KH18 having a 3:1 ratio so I'd recommend that you check this before spending too much more time and money
The time and money
involved may be better invested in a new engine/transmission.