The answer seems to be in your own code ....
OpenTimer1(T1_ON | T1_SOURCE_INT | T1_PS_1_256, T1_TICK);
To change from internal to external clock you need to change the parameters passed to OpenTimer1. I cannot tell you the exact value, you need to look it up in the help for the compiler that you are using, but at a guess changing T1_SOURCE_INT to T1_SOURCE_EXT seems favourite. :)
Hope that helps :thumbsup: