65 atlas replacing RM19 stator with an RM21 stator

I'm installing a new harness on my 65 Atlas (with magneto) and I want to replace the original 3-wire RM19 stator with a 2- wire RM21 but I want to maintain the original lighting switch. Now I have 3 wires coming down to the alternator but only two leads from the alternator. Any tricks on using the RM23 but retaining the original lighting switch?

Thanks

Dave