Front end swap

[The Buddha]

90 kat rear wheel = GS rear wheel. Just use GS sproket drive and all from GS.
FE - 2-3 ways to do it.
You have to take off ~1/2 - 3/4 inch off the top and move everything down.

You can machine the bearing area on top, push the bearing in deeper, then space it to the adjuster, then spacer on top of the adjuster and top triple. That will leave your ignition lock not reaching the neck lock area. So no neck lock.

I have a machinist that will machine it and cut threads lower, so you push the bearing deeper, and put that adjusting collar right there. He also will take out the top threaded insert, take off some off the top, push it in deeper and weld it back up and machine it smooth. That way, no spacers and neck lock fits like its supposed to. Its just 5 more mins past machining the bearing area for him when he had it jigged up.

The guages - If you want to run stock GS guages - You have have a way to bolt them on.
I dunno what others have done, but I slice off the bolt ears off the GS triple, get them welded on the Kat triple at the exact distance they are on a GS. That will let you fit the grommet and fit it like it was a GS ... Now a word of caution here. Welding lacquered aluminum, turns it brown and shitty looking. I have typically powdercoated the thing after the weld job, else it looks like sheite and its very visible.

Then I have spread the ears on the headlight and Turn signal carriers. Welded up the gaps if they open up and run stock GS grommets nicely gooped up with wd 40 so its slick ... and slide it on the uppers and then fit the headlight ears in them. It feels tight, but it has held well and not ripped on any of them ... yet. If it goes, replace with the same parts off a GSX1100G 1992-1999. This step is only for 89-02, Fairing GS = no headlight carriers.

Then finally, you will need longer clip on tubes - the brake banjo will clobber the dash ... just like it does on a GS with clip on's.
Yea ... you do this modification right ... you'd have the same problems as the GS does when stock ... that's when you know its done right. ... 
Anyway, I have a lot of these done waiting for various people to pick up their parts etc ... so want any of these, ask and I should be able to help ya.
Cool.
Buddha.

[The Buddha]

Well you can escape the machining on the neck if you remove a stem off a GS and put it there at the right spot. However be very very very careful there, you can end up with a stem that is sunken in the triple and it will in that case depend on just the weld to hold it there. very risky IMHO. I would like to see the stem stick out under the triple, then quarter it with a cutting tool, then flare it out then weld it in if you are doing a stem swap. It will be held in mechanically and the weld is just an added insurance. Heck you can grind and push in the stem on the kat itself 3/8th inch - 1/2 inch can be used up easy ... then just 1/4 inch on top is needed to be taken up.
However you will have to replace the bottom bearing. It cannot be removed and reinstalled reliably. I think it may be cheaper to do straight machining. Atleast it is in my case and my comfort is more.
Cool.
Buddha.

[SIKDMAX]

Thanks Buddah for the info, looks like I wont be doin it now cuz toolshed sold the bike I was gonna be doing the swap on here in FS.

[The Buddha]

Well you have a GS already right ... just do that one, and sell its FE off, You'd prolly make more $$ by saying its a direct swap for any year GS.
Like I said, a fork seal job is way harder than a FE swap. Actually a kat FE swap + Kat fork seal = GS fork seal job. Why

The removal is the same cos you're pulling the GS FE off, the extra step is taking the lower and upper triples off. That is actually easier than the special tool to split a GS fork leg. Kat fork legs dont need special tool, then doing seals = even stevens, fitting the GS leg back together = fitting the modded kat stem on GS. Then the reassembly is all the same.

But remember though I am not counting machining, welding, brake bleeding, making handle tubes, headlight ears etc etc ... Like you have done all that before hand.
You have all that done ahead of time, then its the same amount and less complexity compared to GS fork seals.

Cool.
Buddha.