Wow. I was going to refer you to the ChipKit forums, but they look pretty dead 
Generally, create a new "variant" directory and "board" for the chip, copy in the source code for the closest matching chip, attempt to compile "Blink" and attack errors one at a time, proceeding to increasingly complex example sketches until you have "enough" working for your purposes.
(well, it looks like the 170 is about the same as the 270, only without USB. So perhaps removing the USB code would be the first thing to do...)