The length of the wire is pretty much negligeable, but the contact at the terminals is critical, these terminals are just crimpd onto the ends of the wires from the factory and for the most part this works, but in some cases the crimp is on the insulation and not the wire inside or just barely making a contact. A much better way to do the terminals is to solder the wire to them, that way there's no issues with the wire crimped too loose, on just the insulation, and there's no issues with corrosion degrading the contact.
There's also the issue with the magneto and it's ground contact, what happens here is the steel core of the mag coil has a thin coat of varnish on it then the ground contact terminal goes on one of the screws that mounts the mag coil. A star washer can usually cut thri this layer of varnish, but to ensure a good contact, you can remove this terminal, sand this part of the core to ensure there's no varnish in the way, then make sure the terminal is clean and reassemble, This will eliminate any issues with grounding.
The next problem area is the plug wire and that cheesy plastic boot, this thing usually breaks inside preventing a good contact, so now a spark needs to jump across from the end of the wire to the spark plug, then jump the gap on the spark plug for ignition to take place... Not good for the cdi unit. The fix here is to use an automotive type spark plug terminal and boot. You'll also need that little cap that screws on the end of the spark plug. Another one I've seen since the wire screws onto the factory plastic boot and into the cdi unit is that the wire can be screwed into place but the screw in the boot or cdi unit went into the insulation instead of the copper core which will prevent a spark, if you got a new CDI and still no spark, and you know the magneto is good, it's worth checking.
Now for the spark plug.. they light the fuel mix, that's all they do, they don't almost light it, and more expensive plugs can't light it any better, harder, or faster. There is absolutely no reason to run a resistor plug on these since these ignitions aren't that strong to begin with, we don't want to be adding resistance to the system. Resistor plugs are for cars and trucks and were made to avoid radio interferance. This is an even bigger problem on newer cars because this interferance can interfear with the ECU... But on these engines, there's nothing that this interferance will effect so there's no need to run resistor plugs.
Irridium plugs also do absolutely nothing for performance, they light the fuel, the same way regular plugs light the fuel, neither one will make a hotter spark, and it wouldn't matter anyway, a hotter spark will light the fuel just the same as a not so hot spark, if it lights the fuel and doesn't miss, it works. Irridium plugs were made for the auto manufacturers so they can offer 100,000 mile tune up intervals, but the word "irridium" it sounds cool, it has a nice ring to it, and the spark plug manufacturers know that, very well. Doesn't it sound cool to say "my car uses Irridium plugs"? Yes it does... so what do the spark plug manufacturers do? You got it... they mark up the price and call it a performance part. The truth is that any decent quality plug will light the fuel just the same as one with platinum or iridium on the end of their electrodes. The ONLY difference is that a regular plug is good for about 10,000 miles or so before the electrode errodes away and starts to cause miss fires under acceleration when cylinder pressures are at their highest. A platinum plug will last about 3 times longer, and an irridium plug will last about 10 times longer, that's the real difference and nothing else.
Just use a good quality non resistor standard plug and you'll be just fine.
