using images as text characters in after effects I've made a series of comps each representing a letter from the alphabet.
Is there an expression or plugin where I can convert the relevant text character on screen to be replaced by the appropriate comp.

e.g. I've made a.jpg b.jpg c.jpg.

I type "abc" into a text field, apply a magic expression/plugin, and the letters abc are overlaid/replaced by my comps?

I'm not expecting perfect kerning, spacing etc.. Just a quick way to get a start on this.
The Characteristic plugin for After Effects is built specifically for this:

https://aescripts.com/characteristic/

You can set  kerning and spacing parameters for each character. Check out this tutorial to see how the font is set up:

https://vimeo.com/72438545

I hope this helps your situation!