Is that black stuff a dry soot or an oily soot? That will help determine the problem. Also, when was the last time that the valves were adjusted?, if you have one of the exhaust valves slightly open, that could be the cause. Have you done a compression check?
Does it use an excessive amount of oil? How old are the distributor cap and rotor, or plug wires? It's hard to diagnose a problem from a picture and not many specifics. The fouled spark is a symptom of something of something else, you are going to need to do more investigation.

Thanks for the pics, makes it easier to understand the differences.
The so called short pulley was used with the water pump installed for factory A/C, IF I remember correctly 21010-H7227. where as the other one was used with the standard pump, 21010-H1027. As a pair they are completely interchangeable, just use the fan with the correct bolt spacing.
BTW all of this is useless not knowing where he is.

First off, WHERE ARE YOU?. Next which pulley do you need, there are at least two different ones. One has a small center hole and the fan bolts directly on top of it, the other one is for fan clutch use and has a larger hole.
If your not sure, check the tech wiki.
I have both, somewhere.

Looks like a custom color. Any good auto body supply house should be able to tell you for sure, and if necessary match it for you.

PM sent

Here in Utah it's now $1.79 per gallon, up from $1.65 a few weeks ago.
For the first time in decades there is not one oil rig drilling in the intermountain west, with oil priced in the low $30 range no one can afford to drill anymore.

David, Actually the SD22 that came here was a decent motor, just anemic like a lot of normally aspirated diesels. Nissan didn't certify a turbo version here but there were companies that did conversions for them that woke them up, made the fuel mileage better too.

Short answer, NO. The pin diameter is larger on A13, A14, and A15 engines than it is for the A12. Check the Tech Wiki for more details.

Not knowing where you are or what the radiator you are using is out of makes this kind of difficult, but...
...I hope that you live somewhere that you have a competent auto parts store close.
Any good parts man who knows their stuff should be able to get you the correct cap, tell them that you need one "for coolant recovery" that will fit your radiator. The correct cap will take care of all the rest. You don't need to change your tank. The type of cap you are looking for will have a rubber gasket on the center section and another rubber gasket around the outer edge of the cap where it goes over the widest part of the radiator filler neck, other than that it will look just like your old one.

There is also one at the backside of the block, behind the flywheel. Like DD said change them all, it may be a lot of work but cheap insurance.
As far as the radiator cap, the later ones were designed for coolant recovery. What you call an expansion tank. This is actually a good idea because as the engine heats up the coolant expands and flows out the small tube at the radiator cap fitting, instead of just venting that fluid to the ground and wasting it, it flows into the tank where it is stored until the engine cools off. As it cools it creates a vacuum in the cooling system that causes the fluid to be sucked back into the cooling system.
That way the cooling system always stays full, and unless you have a leak, you should never have to top it off.
Probably the best idea would be to pull the engine, that way you could get much easier access to all of the core plugs.