Front fork replacement - interchangeability between same brand, style and age?
 

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?

Brand, style - whatever you mean by that - and age isn't the critical features.
For the fork to be able to mount to the frame, The features you need to match are steerer tube diameter and length. To be able to reuse the headset and stem you need to match quill stem vs threadless/aheadset stem.
For the ride characteristics to remain the same you need to match the axle-to-crown distance. And if you're using rim brakes, both forks needs to be made for the same wheel size.
Sticking a sus fork in a frame not intended for that is a so-so thing. It'll raise the front end and change the handling. IME the bike will remain rideable, but you might need a few minutes to acclimatize. From a technical perspective, the longer fork will stress the headtube/downtube joint more, which might cause problems.
In 1" steerer you have the option of ISO or JIS crown race. Can't really tell how probable it'll be to encounter the rarer one.

Thanks for the info. I'm buying it from someone who knows nothing about the bike other than it's missing a wheel (which I have spare). So getting any more technical info is almost impossible.
Both bikes take 26 inch wheels and use rim breaks so that's a start.

I think I have some adaptability in my handlebars to counter a change in the front height.

Let's see how it goes!

You need to think about angle too. The longer the front fork, the slacker the fork angle, which influences how the steering feels.
(There's something called "rake" too, but let's leave that for now....)