Have the same gearbox
setup in my boat WITH the same problem. THE shop to fix these is ANTHONY KEATS here in Ontario Canada
. Their technician with a good reputation by the way figures its thrust washers and a disk wear out problem. The bottom line is it is fixable. He will rebuild
the clutches end to end for about $1300.00. You can not get a replacement unit new as it is a legacy item(obsolete) but the parts
are still available. Check your linkage travel first. You need 35mm or 1 1/2 inches from neutral to fwd or to rev. Good luck...I am pulling mine for overhaul
when the snow reduces. Fyi box has been with this boat since 1977.