Be worth trying putting oil in, drop it into neutral and turn it over with the valve lifters down, on the winding handle, then after a couple of fast revs (by hand) pop it into ahead and give a few more revs and then astern for a few revs.
If the prop turns in each direction without any outward signs of trouble, start the engine in neutral, run ahead for a while and then try astern and see what happens. If you are alongside, you can hard against your springs for a reasonable time to convince yourself that all is OK (or not

)
If it looks encouraging, suck the oil out of the gear box and strain it through some kitchen roll paper and look for metal flakes. Small quantities not a problem probably, teeth a bit more of a problem)
Refil with clean oil and try again
Hopefully it will be OK!!
You do see these units come up on ebay now and then
Where are you?