I'm completely new to bike working and I fully acknowledge that I know almost nothing, but I am trying to learn. I'm mostly working on this bike for the fun of learning. I know it's not worth the money to upgrade it, but I prefer to think of it as a school fee. I plan to replace everything, just for fun.

I have a Giant ATX 850. 16inch frame. Probably mid 90s but I am struggling to find any real info/specs online so I'm hoping my question isn't too basic.
The front suspension doesn't work. I'm not sure if it has ever worked this century as lifting the rubber guard just shows a pile of rust. I want to swap the front fork out for a working one but it's quite the task for a first time project.


I found another mid 90s (I think) Giant MTB Team World Cup to use as a donor bike looks like this one.

How safe is it to assume that the steerer tube will be the same diameter? (ATX is 1 inch)
If so, would that really difficult to remove piece (crown race?) Also be interchangeable?
The donor bike has a 17.5 inch frame. I assume that means that the steerer tube will be longer. When switching from non-suspension 16" to suspension 17.5", how noticeable will this be? Is cutting the tube going to be unavoidable?