I am trying to port code from AT&T lex that uses yysptr and yysbuf.

Those are internal variables pointing into the AT&T scanner’s input buffer. I imagine they’re being manipulated in user versions of the input() and unput() functions. If so, what you need to do is analyze those functions to figure out what they’re doing, and then replace input() with an appropriate definition of YY_INPUT. You shouldn’t need to (and must not) replace flex’s unput() function.