Wow, that's quite a motor - 4.7kW at 6000rpm. Problem is, you need to drive a current in both the armature (rotor) and the field (stator) and be able to vary current in one or both to achieve speed control. Simply connecting them in series won't be a solution, even if you had a controller to regulate the current.

If you can't mend the original drive, you may be best to fit a modern induction motor and matching VFD. Are you likely to be doing that sort of power at full speed? It would be fairly shifting material.