First off take the hood off the motor and check the wiring, before you do anything. Some of the old Mercury's of that era had wire that rotted for lack of a better word. New wiring harness should still be available, but hopefully it will be okay. If not you could end up spending some money. If wiring looks good, I would then pull the plugs shoot a little lubricant in the plug holes and turn it over by hand to spread the lubricant. Then I would do a compression check, cold with the lubricant it should check 100 + I would think on all cylinders. Next I would put lower unit in gear with plugs out and turn it to make sure it feels good, check forward and revers, then check lower unit for water...pull out flat headed screw in lower unit and drain oil, it should look good, not white or milky (water in Oil), also look for metal shavings. If you have good compression and a good lower unit most motors can be made run without too much major expense. Hope this helps....Those old bass trackers are nice and I would think it is worth the price without the motor if it is in good shape and the odds are the motor will not take any major work if it was running good when put up. My dad has a 74 model 80 hp and we got it back running a couple of years ago after sitting for 20 years, he did have to rewire it due to the wiring deterioration. One more thing you are probably looking a rebuilding carbs and putting in a new water pump at minimum....Water pump would be one of the first things.
Good Luck...I would go for it.
Flintcreek