Your company holds the copyright, but since it is published under an very permissive open source license (MIT license has very few strings attached), anyone getting their hands on a copy can use it. That includes you (the original author). You can modify it, publish it, and sell it, as long as you conform to the license - being the original author gives you no special rights. Most licenses allow you to sell the software for money (but allow others to take a copy and sell it for money as well). So you are legally fine.
But you asked about the workplace. So if you think there is a chance that you would want to return to the same company, some overzealous manager might (incorrectly) think that you stole from the company, and that could cost you a job offer. So you should either inform them (and make it very, very clear that you do this out of courtesy and they have no rights to stop you at all), or publish it in a way that doesn't mention your name.