I got a bike from a mate over a year ago and was going to sort it out and ride it

The frame is a Giant ATX 830, the issue im having is there is a ton of play where the frame meets the forks at the bottom, iv just pulled it apart and there appears to be a piece missing, the bearing is there in the frame but the fork is just a tube with no seat for the bearing so theres loads of space for it to move about. The full sus forks arent the originals.

Can i get a new headset/bearing or something? I cant seem to find anything online..

You need a proper headset press, unless you're very very careful with a block of wood and a hammer.  You run a very high risk of splaying the headstock.

And prepare to bend over if you went to halfords, places like Wiggle and ChainReaction are much cheaper, all you need is the measurements off the bottom bracket.
If they don't have any on it anymore, you measure the shell size (generally 68mm or 73mm) and the spindle length.

If you're replacing the headset, a good tip is to cut about 2-3 inches out of an inner tube and thread it over the bottom of the headset.  Once you've put the forks back in, pull the inner tube so that it's covering both the frame, and bottom race.

Should offer a bit of extra protection againt puttukkus.

Basically, you're doing the same as the picture below, but for free.

I don't get involved with wheel tru-ing as it's all too easy to make it go egg shaped.

Should only cost about £5 at your local PROPER bike shop.

Not Halfords.

New headset, bottom bracket, tubes, brake pads and saddle fitted. My brother managed to get the wheels fairly straight so that'll do.

Had a couple of short rides to shakedown and adjust things, seems ok so far, haven't ridden in over a decade, more used to something with some bhp between my legs so a bit wobbly when getting going lol