First, jumping back from Mojave to Mountain Lion is a big step. There are 6 versions back, from 10.14 to 10.8. Back to 2012. If your Mac originally came with Mountain Lion, you can do an Internet Recovery and reinstall Mountain Lion. But I'm sure you will lose a lot of functionality and that many apps will stop working as they require something new.
But if your problem is 32-bit apps, you should know that Mojave supports 32-bit apps. It is Catalina who will release the support for the old apps. With the Mojave you only get one warning the first time you drive them.
It's probably not a good strategy to keep everything back just to run some old unsupported and abandoned apps. See https://macmost.com/what-to-do-about-32-bit-apps-before-updating-to-macos-catalina.html Chapter1