Gearbox trouble help needed

I have a old mariner 30 hp 2 cyl engine with separate tank for two stroke oil. The last time I used the engine it was fine but after 2 years of sitting outside on the boat the engine would not turn over, it turned out the gearbox was locked up. I dropped off the gearbox and found no oil in it ,put some oil in to free it up and both the shaft and the prop are now turning freely ,they had been locked solid .My problem is now that it wont go in to gear and I noticed that the rubber seal that stops water getting in to the gearbox is damaged this seal is on the gear change linkage, could this be the cause of the oil been missing from the gearbox.Is there a online manual for this engine , and advice where to go next would be great.This eggine I am told is the same as a Yamaha and have been told the bigest problem will be opening the gearbox


Joe