I'd personally strip it down, and replace seals, you may overtighten and damage threads..... but you could drain the oil in the final drive? loosen the nut, plop some gasket goo, (the oil/heat resistant type), around it, and re-torque it.....it may work, if you are lucky. Trouble is leaks get worse, as my gearbox leak did on Saracen, lost all it's oil whilst driving,..-unnoticed, save fore a nasty burning-paper smell, but by then, it was tooo late burnt brake bands..... new /replacement gearbox required!!!! oops!