Scolba, I can't tell you what the rebuild or changeover would take. If your 90 is ready to go, though, and you want to test your friends theory about the lower unit, I have a 1989 70 HP Lower unit here for sale and if you want to bring your motor down my way, we'll slap it on and try it out. It's up for sale, though, so we'd need to do it before it's gone, but I'm only about 90 minutes south of you, so you're welcome to bring it down. If it works, I guess you'll have found a lower unit, too. outboardsanonymous@gmail