Back when they were made we always hung inline 150 Mercs on 20's, I had a 115 on my 18 and it was just fine but could have handled the 150 and I wished I had done that for mid range in swells, the 115 would bog a tad. Keep in mind I think these new DI motors may be a good deal stronger than the old ones too, so a new 115 may be comparable, maybe someone else can add something regarding that.
You might check here for a motor-You will have to email in for a quote on a motor but they are on the biggest buy program this year so nobody can sell them cheaper, it involves dealer incentives for commiting to X amount of motors. You would have to weigh it against how good a relationship you have with a dealer, they can get their feelings hurt if he's a good friend.
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