If Dave's advice doesn't work for you, Simrad
makes an NMEA 0183
to Simnet converter which you can connect to the NMEA-out of the course computer to get fast heading data (if the course computer is a model that has the built-in rate gyro). Then connect the Simnet to NMEA 2000
using the Simnet to NMEA 2000
Adapter Cable, add the Lowrance NMEA
2000 Starter Kit, and that will make the connection to the HDS MFD and the radar overlay will work. Total outlay is about $250 for those materials.
Edit: Oops -- I re-read your post and see that the course computer already has NMEA 2000. In that case you should be golden. The Raymarine control head is not where the fast heading data comes from -- it comes from the course computer so you should be able to make the radar overlay work without anything more than the NMEA 2000 connection.