Re: Spark plug wire: an interesting find
Looks like you got one of the newer kits with the rubber boot and the automotive style spark plug clip that needs the little tip screwed onto the spark plug so it'll snap into place... these are the good ones.
Then the older style stock boots are similar to that NGK boot you got, but of far less quality. The NGK boot comes with a resistor to help supress the radio interference that can mess with other electronics like the ECU. Taking out the resistor and putting in a metal slug would be the right thing to do before installing onto a CG engine, but it's still going to have some little amount of resistance or maybe a poor contact somewhere in the boot. These ignition systems aren't very strong so even a few ohms of resistance can be the difference between a fire or a miss during acceleration where cylinder pressures are at their highest.
Why one worked better than the other after you removed the resistor and replaced with a metal slug I'm not sure, but it should have worked just as well. It could be just a thin layer of oxide on the brass parts from sitting for so long or a bad contact on the wire where it was screwed in but without it in front of me it's hard to tell. I did have a similar problem with my Sportster not wanting to crank over and all it was was a very thin layer of oxide on the brass lug on the ground wire... it looked normal but after cleaning it with emery cloth until the brass was really bright and putting the terminal back on it fired right up when before cleaning it all I got was about 1/2 turn then the dreaded click click click sound and no start.
You could try taking it apart again and cleaning all the metal parts that make electrical contact and trying again, but you also got the better type spark plug boot already...