Yes.
Any 20inch forks will fit.
The forks go through the head tube and headset, the stem anchors the fork and then the bars attach to the stem, so the answer is the fork and stem.
the fork gyro (if have) and stem and mostly important the headset
That's a BMX, and there's not a lot of difference between forks for those. AS long as you get the correct steerer tube length, any BMx fork would give you a rideable bike.
Depends on what's already on there, what's available, and how much clearance the bike has at the fork and frame.
You measure it. A ruler and a caliper are useful.
The main parts are: frame fork wheels (handle)bars saddle seat post cranks pedals
Most will fit. but not all. There are a few different designs on bottom brackets for instance, and they aren't interchangeable. If you look at older bikes the stems/fork can differ too.
To measure the length of a fork you would use inches or millimeters.
The headset is a set of bearings that allows the fork to turn smoothly in the head tube of the frame w/o sticking wobbling around when you turn the handle bars.
It ha a threadless fork with an aheadset stem. Remove compression cap, loosen the steerer tube(rear) screws, pull the stem/ bar off. Then the fork should come off with a light tap on the steerer tube.