You can use
this as a rough guide though I advice you to try to understand what the changes are doing exactly (Note to self: improve that guide). You'll also need to check inside the compiler where e.g.
system_arm_wince or
system_mips_wince are used and decide whether or not to use your new
system_mipsel_wince/
system_mipseb_wince as well. Also you need to find out whether WinCE on MIPS is using a different calling convention then what FPC supports there (if it is you'll have to extend the code generator to support it) and whether GNU binutils are available or if you need to use the Microsoft tools (which will need additional code to be usable). Then you can extend the WinCE RTL located in rtl/wince with the System unit being the most important one to get at least roughly working.
Of course you can ask for help if you encounter problems.