antlr-options-assign-string is a variable defined in `antlr-mode.el'.
Its value is
" = "

String containing `=' to use between option name and value.
This string is only used if the option to insert did not exist before
or if there was no `=' after it. In other words, the spacing around an
existing `=' won't be changed when changing an option value.

You can customize this variable.