PM Nizzbits with full details and get good prices on genuine stuff, OR go to Bursons and order the parts.
You can't put a disc brake axle on it because there's no such axle around but you could try to fit some Terrano discs to your existing axle. Haven't tried it before, might work.
First i would like to welcome you in this forum and regarding your Navara 1989 model for which you are trying to purchase a wheel cylinder with push rods ,.yes you can pull the disc brake differently on it you will be able to fix it or you can take help from a mechanic he will help you get this fixed for your Navara.