trying to figure out the year of my johnson engine

Hi guys. I am trying to fix an old engine between me and my friend but we don't know what year the engine is. I took it a mechanic and the guy told me that it was going to cost me a grand to fix and because the engine is so old he couldn't tell me how long it would last. that's why i am attempting to fix it myself. The sticker that i suspect show the model number, etc appears to be painted over When i bought the boat the guy who sold it to me told me that the engine was a 1980 75hp johnson but i am not sure. Since me and my friend are going to be fixing the engine ourselves i wanted to get the manual for it so it can guide us somewhat. Below are some pictures i took of the engine and the piston i need to replace. If any of you guys could tell me the year and maybe point me in the direction of the manual i should get i would greatly appreciated. thanks.*
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http://i.imgur.com/ZqdtgOG.jpg

http://i.imgur.com/0pvvCT3.jpg

http://i.imgur.com/WBqPmaO.jpg

http://i.imgur.com/GqKnGTN.jpg

damage piston part number "320955"

http://i.imgur.com/NBAYwby.jpg?1

ps. I am looking for that piston so if you know where i can find it even better.*

First 2 pictures it is easy for me to tell that it is about a 1974/1975 model , perhaps 76.-----Looks like a " salt water " motor too.------From the looks of the piston you need to rebore the cylinder and go oversize on one.----What is the diameter of the cylinder now ?----The wiring sure looks awful with 2 big red wires ???------What does the crankshaft and other 2 pistons look like ??

sorry i didnt respond to you sooner. I appreciated your help. I honestly didn't think the engine was that old. The mechanic who checked my engine put it back together minus the piston so i dont know the condition the inside of the engine looks like. However he told me the cylinder was in good condition. I havent measure the piston yet so i dont know the diameter yet. I wanted to get the book first so when i take the engine apart i wont miss anything.

Picture of the cowling by any chance ?-----Have you ever run this motor out on the water ?---------Is this a new purchase for you ??---Big red plug on the wiring harness is on the wrong side of the engine ?----Looks like there are parts missing on the electric choke solenoid too.--Electric choke obviously does not work !----Well , when I look at the power pack on your engine and the shift linkages , I identify your motor as a 74 / 75 model.-Possibly an early 76.-------Check this out yourself with parts blow-up pictures right from this site.-------Pay particular attention to that arm that is at a 45 degree angle !!!----------Your " mechanic " put the engine together with no piston in one cylinder ?????----------------Now there is a chance that this is an older ( 74 / 75 ) powerhead on a 1980 model.---In this case you may find that it shifts into reverse when you put it the control box in forward gear.--------It would be worth your while to check this out.--Good luck with this project.