99 sportster standard 883 front wheel change
 

I bought a 19" 2.50 -3/4" spoke rime to replace the 21" rim. The brake caliper won't match the rotor by 1/8". Is it not compatible or am I missing something?

It is possible the forks on your bike are spaced wider apart that on other Sporty models. A 21" wheel is often fitted to give a custom wide look. You will find our Sportster experts over in the dedicated Sportster section, so I suggest you ask over there, where you should get a prompt reply.