Well.. what temperature was the check of 13 ohms at? Resistance checks on the ignition coil should be conducted at 70°, and I believe colder temperatures will cause the Ohm reading to artificially rise...

Also, what I just found out myself is that if you have an old ignition coil on a Honda, and you crank the engine repeatedly with the dist rotor not making any electrical contact (eg you're cranking your engine to get the rotor in the right position to take it out), your coil can very easily burn out.

But getting a very weak spark and having such a borderline acceptable Ohm reading might be indicative of a toasted coil...