Wow, sounds like my workplace - pretty much the same story, except the two devs who wrote the 300,000 line monstrosity are already gone (worst page record: a page with one topic and 300 comments generates over 10,000 - yes, ten thousand - calls to MySQL, AFTER caching).

In our case, we're squeezing in a complete rewrite of the codebase around existing client work using the old platform. We're writing it as a few separate services (authorization service, content service, front-end, reporting service, and administration front-end) instead of a huge, monolithic app. To do this in a decent time frame (a few months), we're adding 5 or 6 contractors to a 3-person dev team, leaving 3 other devs able to deal with the existing platform.