I've done this a few times. If the headtube length works, you're just swapping out the races with moped ones. you can even use bicycle shit. Are your KX80's the conical needle bearing timkin style? Is that the problem? Old ones were balls I and then it's usually just finding the right size balls or cage.
But if it's the timkins yeah you just gotta knock the race off the bottom of the steertube basically like you were pulling bearings off a crank - like i always just tryn get a narrow chisel under it or whatever, pry it up. then you need a regular ball bearing race like from a moped headset to go in place of it that has the same ID. Be aware that usually the bottom race is flared out a little oversize and not the same diameter as the rest of the steer tube. (ex: moby bottom race is 26.5mm ID on a 26mm diameter steertube, maxi is 27.3mm race ID on a 27mm steer tube) if you're being cheap you can adapt an oversized bottom race and fill in the difference with coke can shims or whatever. i can hear the haters now but it's fine. it works.
Then hopefully the threading on the steertube is standard and you just use your regular moped top race and tighten it down as usual. if not, you can get another bottom race of proper size as an unthreaded top race and use the original KX headset nut to tighten it down. Or again if you're stupid and cheap you can dremel the threads out of a threaded one til it's the right ID and just press that down with the original hardware from the new forks. This set does it that way, 2 bottom races so your top is an unthreaded race : https://www.treatland.tv/tomos-magnum-headset-bearings-p/tomos-headset-bearings.htm
After that it's just matching your new races to the right cups. if ythey don't match the bearings you have, swap the cups wit hwhater does fit. I mean you can probably just buy that set from treats and be good to go, i'd measure the steertube OD tho. especially because usually it's flared at the bottom so the two races have to have different ID's
If the steer tube is too long, you can go with the unthreaded top race and just add spacers under the nut you're tightening it all down with. you can make up a ton of difference that way. If the steer tube is too short, you can press in a longer one, or just cut down the headtube on the frame. usually won't need much, and there's almost always a lil extra, cut it shorter and press the cups back in.