In another discussion we covered which page builders already work with ClassicPress:
"We do have Beaver Builder publicly supporting CP.
FX Builder was developed for CP by forking a plugin for WP and I have it on some of my sites because it’s just GREAT.
Thing to note BB and FX are not compatible with one another but that is not a problem since one of the two is usually enough to develop a site."
fwolf Founding Committee Member
"Re: Page Builders working with CP
Technically, Visual Composer (or whatever its called nowadays) should also work with CP, as it basically is just a nice UI for a shortcode orgy.
There also is some kind of Bootstrap-specific page editor that looked promising a few years ago, but I sadly cant seem to find it at the moment. Still, got its sources saved here somewhere (also, it was on github … I think), so it should be still good.
cu, w0lf."