Modifying Transliterator Rules I'm using the MacOS' Tamil language input tool. However this question can be answered in a generic way. 

I am trying to make some changes in the rules under this folder. 

/System/Library/Input Methods/TamilIM.app/Contents/Resources

How do start? Meanwhile I'm looking to solve the SIP issue by remounting the file system. 

Will manually editing the Transliterator.txt file would work or do I have to compile the binary?
I think you should be able to just edit Transliterator.txt.  I did this once long ago and it worked.  But things may have changed since then...