Dragon Rampant doesn’t really go in for warmachines so there is no direct analagous unit stat to use. Make it a slower (6” move) greater warbeast with no wild charge? Add wizardling with a new spell:
For whom the bell tolls: 7+, all friendly units within 12” receive +1 to courage tests and enemy units -1. Lasts until start of next Skaven turn.
Would cost 8 points.
Alternatively make it an upgrade to a skaven unit. So it has to be paired with another unit and gives a passive +1 courage to that unit and -1 to any attacking unit for + 4 points. It’s removed with the unit if it routs or is wiped out.