Your story sounds very similar to mine. Last winter I also got a boat that has been stored in a barn for 20 some years. The owner was dying and going through belongings and apparently forgot he had the boat. His grandson got it but discovered the floor/deck had rotted from being soaked with water from the leaking roof in the barn so he didn't want to mess with it. I found this boat on craigs list and it was only about 8 miles up the road from me. I went and looked at it and fell in love with it so ta speak right away. It is a 1976 Arrow Glass Cheetah with a Chrysler 135 outboard. All we did was get a fresh tank of gas and it fired right up and ran very well. Of course I did change the water pump impeller right away. I did a compression test and it has 145-150 on all 4 cylinders which is great. I ordered carb overhaul kits along with a fuel pump rebuild kit and am going to change all the hoses. I took the gear case off and replaced the seals on the top side where the water pump is but now want to do the lower seals as well. I can't get the bearing carrier out as of yet.
Your probably going to do the same thing I am doing since it has been sitting so long. I wouldn't take the boat out in the water without doing the carbs and making sure the gear case is sealed up good with fresh oil along with a new impeller. If your mechanically inclined it's all pretty simple really you just need tools, time & patience. It will be all worthwhile when it's done.
Good luck,
Nick