'81 kz550A rear wheel replacement 18" drum to disk

so last week i got cut off on the road and ended up hopping the median and taking out both my
front and rear wheels. i ordered replacements, used of course via ebay... only issue was the rear wheel.
i could not find any 18inch rear drum brake rims so i purchased the only one i could find which was an 1984 550 gpz rear wheel.

what i came to find upon arrival was that the gpz rear uses a smaller wheel coupler than the drum wheel.
i have the rear caliper & bracket off what i believe is either a 650 or 750 gpz that i intended to make work to upgrade
to a rear disk to match the rim i purchased...
i know the gpz has a different swing-arm with a mono shock, i havent even gotten into if the wheel& caliper will fit into my swingarm.

so can anybody tell me if all my efforts thus far are in vain??? if i find the correct coupler to fit the rim i bought have i any
hope of fitting all this onto my bike...??? i know i dont have all the information regarding the caliper but any insight would be
very much appreciated...

thank you from a new member in a bind.

Sorry about your off road excursion, but glad you are ok.
I have not done that conversion, but my first concern would be the axle. Do both 550 models use the same one? Does the brake caliper bracket need the same diameter axle? You will likely need o fabricate a brake stay mount on the swing arm, and will likely need to fab a mount for the rear master cylinder.
Spend some time on Partzilla or Kawasaki.com comparing part numbers between the models. The more in common there are, the simpler this will be.
In the mean time, I am sure some of the 550 guys here will chime in with some expertise .

I would take good photos of the stock rear wheel assy as installed on the motorcycle, then check if a 750E or whatever looks the same. The 750E has an 18" Rr wheel w disk brake. There are at least 2 of those wheels on eebbaayyy right now.

From the diagrams at Partzilla the 81 550A has the same rear axle as just about every 550, 650 & 750-4. any of those complete wheel assys might swap. It seems to me that the axle width & wheel assy width would have to match. but whether or not the drive chain etc lines up is another matter. This really isn't the same thing, but I installed 550 Ltd wheels onto my 81 650-CSR --- it was a direct swap. Both models have some of the same hub components as the 750E.

Swing arm. Does your swing arm have provisions for a disk brake? I'm asking because the 550A diagram drawing seems to show a disk brake swing arm, but that might be an error.

I'd stick with just a KZ550 18" drum. Just wait from one to come up. If you want to switch, only use the 1981 GPZ550 18" rear wheel and brake. Believe me I've battled many wheel conversion's and it's NOT worth the aggravation if your not racing the bike. Call bentbike in Lynnwood WA, or StealsCycle in Littleton CO I'm sure they have one.

I would recommend upgrading to the 81 gpz550 dual disk though, its worth the effort.

If you do a front dual disk conversion to the 81 gpz550, you will need the 81 gpz forks as well. The Kz550A forks look the same but they are not. The caliper mounting point is in a different place since the Kz550A disk is larger. You could put two Kz550A discs on the wheel, but then it will weigh a ton. You might also try to make an adapter bracket to mount the calipers lower. But if yu are going to do all that, I would get newer calipers from a later model Ninja.

And yes, the dual brake conversion is worth it. I've ridden them side by side and the gpz stops much better... faster and more controlled. The rear didn't make a whole lot of difference in hard braking. disc vs drum. Maybe the disc was a little more controllable on back, but I don't recall a huge difference.

Oh, and the factory rear master cylinder for the 81 gpz550 is impossible to find in good condition, and there are no rebuild kits for it, and it was a one-year item, so you would be fabricating a little for that as well (not too difficult, though), in addition to fabbing brackets etc, and looking for a caliper mounting spacer/bracket.