There are 2 hole4s ( 10-24NC ) to mount the pump.-----There should be a gasket.------The hole in the middle provides a PRESSURE pulse to drive the pump.
With a high speed camera you would observe what happens to that heavy prop.----The engine fires ~ 4 times for every revolution of the ---PROPSHAFT----And accelerates that heavy prop 4 times.----Not visible to the eyes at normal speed.
You took out the pivot pin for the shifter.-----Not the same as the drain screw.-----And carefull getting that in as stripping the internal thread will be a big issue !!!
Ran at 24:1 in those days.-----And yes that is normal for that vintage motor.---Yes some gas and oil goes right through a conventional 2 stroke and is wasted.-----There may be things that can be modified to reduce the sludge.-----Not quick and easy.-----
When it was new it just slipped in.----But now 40 years later the corrosion product have jammed it.----Bigger hammer or visit your local boat shop.----In Canada-----evinrudeparts.ca-----May assist finding parts.
Everybodey works to different standards I guess.----And sorry ----I can't believe there was such an issue in the repair of these PRESSURE operated simple pumps.-----The only thing that can fail is the diaphragm.-----An item that is easy to make !!
Changing parts can be an expensive way to fix a motor.----Many years ago I determined that many folks have trouble with their small engines.-----Simply because they are not using the right procedure for starting.