On my boat I have twin 2 stroke Yamaha's 90 models, I had a leak to, Problem is due to the age of the motors there could be a problem with seizing/corrosion on the prop shaft area you need to work on and you may not be able to take of the prop shaft to put a new seal on , This is what happen to mine, Mine is only very small leak , Your leak seems a lot worse that mine, And yes I would be getting someone who does this type of work all the time , Cause if you stuff it up and hand it over to someone to fix you look like a fool, One more thing make shore you can get the part first before you start pulling it apart.