Pages won't support scientific equations (MathType) so 30 years of work is gone!

MathType for Mac: https://www.wiris.com/en/mathtype/iwork/

Add mathematical equations in Pages on Mac - Apple Support

Note (in that article under Add an equation with MathType): “Tip: To set MathType as the default equation editor, choose Pages > Preferences (from the Pages menu at the top of your screen), click General, then select the checkbox next to “Insert equations with MathType.””

The current state of MathType by Wiris is that it no longer supports Apple's Pages application and is a subscription add-on solution for Microsoft 365/Office 2021 for Mac restricted to Word and Powerpoint applications. There is also a MathType add-in for Google Workspace (Docs and Slides), and a Windows solution.

So, unless you have Microsoft Office with that MathType add-on installed, your older documents using the Design Science vintage MathType and its Math fonts are orphans on newer versions of macOS that no longer support the old MathType. One option is to export those older Pages documents (and with the old Design Science MathType installed on that older operating system) to either Word or PDF, and try opening either of these in a current version of Word on Monterey with the Wiris MathTYpe add-on already installed.

The equation editor in the current Pages expects a limited subset of LaTeX math syntax (or MathML) which it will convert to embedded MathML objects (that are portable to MS Word. Only practical in the context of new equations, and not the herculean task of migrating your existing documents.

Have you tried exporting one of those texts to Word format and seeing it you can edit it with Word for Mac?