Reddit mentions of Lisle 20610 Inline Spark Tester

If you're uncomfortable with testing the spark plugs when pulled from the cylinder, they make an inline spark tester that can help verify that the sparkplugs is firing.

https://www.amazon.com/Lisle-20610-Inline-Spark-Tester/dp/B0002STSC6

sometimes you have to put it right on a steel bolt off the frame/engine/wherever to see a good spark.. but it's usually not that hard to see. Another option is to get an inline spark tester... https://www.amazon.com/Lisle-20610-Inline-Spark-Tester/dp/B0002STSC6
harbor freight does have one.. should work well enough. Good to have regardless.

If you're confident it's not sparking, First replace the plug and try again. it's the cheapest and most logical possible cause. Next would be battery.. sucks because it costs a fair amount of money. Then the coil under the gas tank.. if battery and new plug don't fix it (still a weak or no spark). That inline tester would be pretty handy at that point but if you have $65, get the cap on coil replacemnt/upgrade that SSW makes.. http://www.sisnerosspeedworks.com/dr-z400-cap-on-coil-conversion
It's not an upgrade in the sense of performance or anything.. but it gives your bike modern tech and will almost guarantee that none of these are the issue you are having. Really, replacing those three parts is good for you to do eventually anyway.. the coil is not so necessary unless you have an issue, but the other two are pretty much a periodic thing you would do eventually . So essentially you really only spend on the coil where you may not have to. If none of that fixes it.. your harness has an issue somewhere and get a meter out and start measuring. I think that's the route I would go.. of course I'm trying to isolate this over a keyboard on reddit so I reserve the right to be a little bit off on my recommendations! Hah

Where to begin.

First off, never again put a meter lead on a spark plug wire. I have no idea why that meter didn't stop working forever. You are hitting the meter with 30,000 volts when it's probably not designed to measure over 1000 volts. You check for spark using a spark plug, or one of these: http://www.amazon.com/Lisle-20610-Inline-Spark-Tester/dp/B0002STSC6 and a video how to use it is here: https://youtu.be/eQBtzk-dGYM

Second, Your starter clutch is fine. It's allowing the starter to turn the engine over. That's the first function. The second function comes into play when the engine starts, and you haven't gotten there yet. IT allows the engine to run without the engine driving the starter motor. The thing you see moving in that window is the flywheel, and it's connected to the crankshaft, so it's going to spin whenever the engine is turning over to start or when it's running. Cover that hole up. your problem isn't there.

Third, the little philips head screw on your carburetor is a drain to empty your carburetor float bowls for winter storage, or to set the float height. It's not going to help you start the engine, although the fact gas is coming out of it is a good sign that that carb, at least, is getting fuel. But, tighten it down and leave it.

Fourth, and this part scares me, is how do you know the left carb needs a vacuum diaphragm (as they're more commonly called)? Good on your for being able to repair your starter motor and get it back together so it turns the engine over, but it's very easy to screw up a carburetor without even knowing it if you don't know what you're doing.

You need to get a socket for your spark plug, they need to come out to see if they're wet, or fouled, or even have a proper gap.

I could go on forever, but go HERE and start reading. You could go straight to chapter 13, "Trouble Shooting a Bike that Will Not Start," but you'd be doing yourself a favor by starting at the beginning and reading everything.

Good luck, man. I know you're trying and I commend you for that, but I saw so many things in just that short video that made me cringe. I don't know where you live, but if you live anywhere near Syracuse, NY, I could probably help you get that running in short order. I've repaired bikes professionally (and in fact am a very busy industrial electrician/mechanic,) and was a service manager at a Kawasaki dealer for a while. I've been repairing motorcycles on and off for over 45 years, since I was 13, and fixing mowers and chains saw even before that. I know what I'm doing and if I come across something I don't know, I'll find out and won't bullshit you in the meantime. For example, the guy who asked about valve adjustment is not wrong, he's just jumped too far ahead. The fundamentals of a viable engine are fuel, compression, and spark. It seems like you have all 3, so it's probably something relatively minor preventing you from getting started.