I have a 1989 johnson 50 hp. The lower unit has developed a knock in forward gear and I found flakes of metal in the oil. I am a auto tech by trade and can read a manual pretty well. I have never been into a lower unit of an outboard before. Is this something I should try. I mean has anyone rebuilt theirs? Any tips? Most people are telling me to just buy a used one. The price of a used one isnt bad but there is really no way of telling it is good. Let me know what you guys think. Thanks.