Redefining a WordPress core function that might be used all over the place by the core, the theme or plugins isn’t a very good idea in my opinion. Because it might be used in those occasions expecting the default value to be what default is, which might lead to breaking things. So it is actually not just not a very good idea, it is a terrible one – in my honest opinion.
Besides I don’t quite get why you can’t just call/invoke the function, in the cases you need the second argument to be not the default, with the second argument declared to be what you need to get the output you want, like this:
wp_function( $arg1, true );