If you did break the selector link then you won't be able to get a spare part. But I did create an 3d model for it so you could print it out. Preferably in aluminium otherwise abs plastic could also work. This is my first prototype and you may need to shave some edges off.

https://drive.google.com/drive/folders/1...

George is absolutely right DO NOT DO THIS because your link will break since the metal quality is quite low.

The motor is highly magnetic and can sometimes take the loose screws with them. Be sure to keep those away from the motor.

It can sometimes be a little hard to get the pin out especially when it is your first time. Try to use some WD40 to loosen the screws up if necessary.

If you can't remove the upper receiver then the charging handle that is attached to the bullet ejection part may be blocking it. Try to pull and hold this back while sliding the upper receiver forward.