I cannot answer your question.
The '90 KDX200 indeed has a single piston front caliper. The 'H' model KDXs have a dual piston caliper..that's part of what makes that particular swap easier.
I would be surprised if you could bolt up a single piston caliper to a dual piston caliper bracket (on the fork), let alone having the rotor work.
Although it's not always true, you should check part numbers on the buykawi website. The 'not always' part meaning that just because the part numbers are not the same does NOT necessarily mean the parts are not the same. BUT...if the numbers ARE the same...you're good to go.
Good luck!