I believe you have MS4A gear boxes.
Volvo Penta doesn't manufacture these any more and hasn't for years, but most of the parts are still available. They are a bit tricky to rebuild however.
What Volvo Penta did was replace the MS4 gears with MS5 gears and then replaced them with HS1A gears.
Other people have replaced these gears with ZF63A gears, but this does require a bit of adjustment on the engine beds and angles and such.
If you supply me with the list of parts I can give you a quote for these parts. Incidentally MS4A schematics are on our web site in the schematic section.
Or you could send us the gear and we would examine it and give you an estimate for rebuild. Most of the parts are available, but if the input gear has been chewed up, it may not be worth rebuilding.
Volvo Penta still produces the HS1A gears to order.
The ZF63A are also available new, but you have to make sure that you can get them to fit without
to many issues.
Hope this helps and please feel free to contact me if you need any more information.
J. D. Neeson