In my tiny little mind, there are a few “rule of thumb” things to do with those green tank Mercs.
One is look at the coils and power test them on a Merc-O-tronic or Stevens coil tester. If the results are the least bit unfavorable, junk the coil because its only going to cause you heartache later. Sometimes there is evidence that someone has tried to repair a cracked coil with epoxy. Be very wary of using such a coil in a motor unless you enjoy being stranded somewhere.
Secondly is go through the fuel system a million percent. As in, clean the tank out. Then clean it again. Clean it about eleventy-seven more times, then clean it once more. By then it should be OK. 
Same with the carb but you only have to clean it eleventy-five or so times. Or once, if you use an ultrasonic cleaner and a good detergent or solvent.
Thirdly is expect that the crankshaft oil seals are going to need to be replaced. Top and bottom. Motors that start up then seem to magically fail for no apparent reason are often the work of old seals that don’t anymore.
Hope this helps eh.
Best,
PM T2
He's livin' in his own private Idaho..... I hope to go out quietly in my sleep, like my grand-dad did..... and not screaming, like the passengers in his car...
