hi viper,
that switch will work with your gx670. i have 2 honda v-twins and have used similiar switch. The connections to your switch are listed below.
M-mag. ground(wire this terminal to a solid metal ground. When your turn switch to off position, this grounds out the magnetos.
M-Magneto, this term goes to the magenetos, usally to the common side of shorting diodes.
S- solenoid, this goes to your electric start solonoid on your starter motor.
I- ignition, this goes to your fuel cut solenoid on your carburetor
B-Batt. This goes to hot +12V,Should be fused 25A.
C-this for electric choke, probaly not needed in your gx670.
Below is a link to your switch details on seadog site.
http://assets.seattlepub.com:8020/Su...g/TechSheets/5