I find very little info for a "Mantis 20"; and am surprised it would have CDI. What makes you think it is CDI? Maybe post pix of the engine/ignition components. Does this coil have two windings (primary and secondary), or is there only one high-voltage winding?
Secondary maybe 10K ohms, or more. The primary will only be an ohm or two, too low to measure accurately with a common multimeter.
Does this have breaker-points; have you chased that black to find out what it connects to in the crankcase? Where is the flywheel/magnet in that picture?
To me, "CDI" means "Capacitor Discharge Ignition", much more complicated than what appears in the picture, and much more expensive than justified for a small engine like this. Again, why do you think it is CDI???
Well, I read-up enough on Kioritz/Echo/Yamabiko engines to believe that is CDI, not Kettering. I better leave you be. I have no clue what all the bits and pieces of that system are, much less what typical coil resistance would be.