You can only use them on the G1 bodies.
The only modern Headmasters so far are the ones by headrobots:
- Modern Hardhead: Headrobots Hothead standard version and comic version, mounts on Universe Hardhead (Universe Onslaught repaint)
- Headmaster Onslaught (it wasn't a headmaster in G1, however...): Headrobots Butcher, mounts on Universe Onslaught or (better) Henkei Onslaught
- Cerebros/Fort Max: here it is a bit tricky... Headrobots made a TFCon exclusive named "Stronghold" with a Headmaster (a black repaint of their Hothead release) and a sticker set from reprolabels with a strong Fortress Maximus feel. Then Headrobots made a set named Centurion intended to update G1 Cerebros. I managed to combine the two sets on a Universe Onslaught in order to have a classicverse IDW/Marvel Fort Max. I've spent LOTS of money (both sets go for 50$ each) but I'm very very happy with the results. Of course it only works if you are a Fort Max sucker but don't need him to be 2 feet tall.
- Headrobots also teased "Darksol" (a kit to turn ROTF Mindwipe into a classicvers headmaster Mindwipe) and "Savant" (which turns DOTM Skyhammer into a classicvers headmaster Highbrow). If you are interested in these two and don't own the two figures that have to be update, hurry up because prices are skyrocketing.
I hope I've been helpful