My Discourse Standard Install – Literate Computing, LLC and Discourse Pro Install – Literate Computing, LLC include the math plugin. If you choose not to use the install in the first 30 days I’ll delete your site and refund your money. Also the free trial includes it Plugin directory | Discourse - Civilized Discussion. If you already have discourse installed, it’s a safe plugin to try out.
Edit: oh, but you have a bigger problem to sort out. Discourse Math Plugin. I’d recommend setting up a staging instance to see how your old data will work with the newer plugin.