Header post injection plugin recommendations

What are your most recommended plug-ins for header injection? My client's WP has a theme which has (somehow?) disabled the CF abilities.

So I am needing a plug-in that enables me to be able to inject into the header. Focus being: I need to be able to do custom injections for individual pages. A lot of plugins out there are for the whole WP, whereas I need to be able to do this for some pages but not others.

Any reccs would be appreciated!