1979 120hp mercruiser I/O

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I have just purchased a 19' 1979 Chapparral I/O with a 120hp 4-inline Mercruiser. THe boat, hull, trailer, etc. is in pretty good condition. THe engine is only firing on three cylinders but I figured that I got it at a halfway decent price at $1800. I have rebuilt a couple of motors in my time and figured that this little four banger would not be too much problem. But I don't even know where to buy a rebuild kit. The engine SS# is 5087730 (built in DuFond, WI). Is this a Chevy 153 cc motor? Also should I buy a marine rebuild kit or will a regular automobile engine rebuild kit be o.k? Anybody know what size engine I have? I would really appreciate whatever advice that I could get. Thanks

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I would stick with marine parts. Yes, the engine is based on Chevy block, but there are differences.Here is one place to find parts online. http://www.knightmuzzleloaders.com/