Does the software to conduct a 51% attack exist?
When you have open-source software, it's a fine line between "configuring" and "writing". 

Say you want to reject all attempts to spend a particular transaction. There isn't a command-line option or config file directive to do this in the standard Bitcoin Core distribution, so in that sense the answer is "no". But you can do it by adding two obvious lines of source code and recompiling, which is only marginally work than modifying a config file. 

There is certainly no need to rewrite Bitcoin Core from scratch, if that's what you're asking.  You can assume that if anyone is planning a 51% attack, the need to modify software is not going to be a significant obstacle.