Get a parts diagram, any 80's 50hp. will work.
Look at the "gear housing" diagram, you can possibly find one on this site?
Your tube has a bend that kinda goes into a "blind" hole.
Flashlight, look up inside and try to find the hole and push the tube up into the hole.
The "hole" has a seal and a washer, just hope the washer and seal isn't in the hole crooked or missing.
I bought a snake camera for 11$ for my Android smart phone.
Try a camera up inside and see if the hole's blocked.
If you can get it in and it doesn't fall out?? then the pressure from the lower will hold it in place when all is tightened.