There is nothing in sys.txt that 'activates' TTS. It depends on the proper files (voice and UX modules) being in place and a proper TTS engine installed in Android for the options to be usable in Settings.
But, if you are curious, below is my sys.txt. It will not solve your problem.
Spoiler: Sys.txt
Attach the *voice*.zip file you are trying to use, and I'll have a look at it to see if it is suitable.
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