disk brake options (factory) 73 Scamp

You need factory stock 73/later "stuff." --for disk brakes. You need upper a arms with the large balls, the disk specific spindles, and of course calipers and rotors also 73/ later. The drum spindles will not work. I'm thinking you also might have to change the lower ball joints

Disk were optional, don't remember if/ when they became "standard equipment." I have a 74 Scamp also drum, also small pattern

Also drum cars do not have a proportioning valve, only a drum type distribution block. You'll need to either replace the distro block with the 73/ later style valve, or do a bit of line re-routing and add an aftermarket prop valve of some sort