Hello V,
it IS possible to use port1 or port3 pins for I2C-bus. I have seen it many times in schematics, and I used them for it in my own projects, too. Look at the datasheet of your controller, you will see port3 and port1 have a FET at each port pin, with a weak pull-up internally. All you have to do is make the pull-up somewhat stronger, by adding resistors from the port-pins to supply.

Henk