Migrate MOSS 2007 (Fab 40 Templates) to SharePoint 2019

It´s high complexity question, but possible to migrate installing all wsp with compatibility level (for example , using Install-SPSolution with -CompatibilityLevel parameter), but without upgrade the UI Experience (I don´t know if ShareGate 3rd party makes upgrade without upgrade UI Experience). But, I think, in some moment, you will need to discontinue the Fab40 (as you mentioned, from SharePoint 2013 to later do not have some templates that support Fab40).