You need to use the values that are given in the manual for the DEFINE OSC, otherwise, you will just get errors trying to compile. What microcontroller are you going to use with the MCP2120? It sounds like you are trying to share the crystal? That won't work at these "weird" values. But with a separate crystal, you should be able to get to the baud rates you need. There are some newer chips that can run on internal OSC at fast baud rates.