I have worked on lots of motors.-----That amount of rust indicates water / moisture was in there.-----How it got in there and over how many years is hard to tell.
Those wee handy pumps come with a 3/8-NC thread.-----Very handy to pump oil into the lower unit.----That would be the most common one on the boat shop shelf.-----Not hard to find.
They had a production run of X number of motors.-----Sold the same motor for a number of years.----Then a new production run.------Not much different than what others did over the years.
50 parts gasoline for 1 part oil.-----Seller likely is flipping the motor if he does not know ratio.----Or if he has been running it then did he do damage ?
Turn key to on.----Push in and listen for a click.----Use an assistant to push in key.-----Use screwdriver on enricher and near your ears to listen for " action "
It is bulged around the hole for the pin.-----Simply file the bumps around the hole and bearing head will slide off.------Use a dremel ( carefully ) to weaken the seal and it may fall out.
If we go by the identification plate then this motor has had electric start and battery charging installed at a later date.----Identification plate ( 1983 ) is original I believe.----An electric model would have been J10ECTR
If that lump of metal is a 20 F you will have to hope it starts and warms up.------Water might freeze in less that 1/2 hr if motor sits in open while you figure it out.-----Best wait for warm spring.