Hi Rob,
5.3 ohms is good for the coil so they should be alright. Have you tried powering the game on with the J1 connector to the CPU disconnected? It may tell whether the problem is on the magnet board or CPU. Did the diodes on the magnet board also measure OK?

It looks like the magnet board is connected back to the 6821 at 11B on the CPU board. The best way to test the 6821 use Leon's DE test rom and use a logic probe to check the outputs of the suspect 6821 are pulsing. If you have to remove the 6821 from the board, you need to be very careful as there is very little clearance around the holes for the pins and tracks which run in between them.

regards Derek