I always did it the redneck way and pull the plug, stick it right back in the boot, hold it to the block, and cycle the engine (be careful, hold the boot with insulated pliers and gloves, crap that hurts). If you don't want it to run because it would be a danger to you or risk damage to the machine you can pull the other plugs and boots and test. It could be a switched wire, sometimes different engines use unusual wiring or cylinder timing and firing order, a couple lines can get crossed and one might work so it hides the problem pretty well. The advantage of this method is that it'll tell you a lot right there and now with all the components of the system being tested, you'll quickly be able to determine how strong the spark is by actually seeing it, and as the engine cycles you'll be feeling the compression and smelling the fuel air mixture, really getting a feel of what's going on in a hurry. I was taught that if an engine has fuel, spark, and compression it has to run.
If it's sparking you got fire, on to the next system. If no then check the plug and make sure it's not shorted out, check the gap and the plug for clues on how it's been running, and clean it well and try to spark it again. Check the plug boot to see if it's corroded, loose, or broken. Then check for volts mainly to see if you're getting anything out of the coil at all, and then check what kind of system it is. I've seen some that rely on a magnet in the flywheel to provide the spark, and rust/ dirt will get on the flywheel and clog up the works, or the gap would be off. A contact point with the spring type ones could be dirty or have a connection that worked loose or corroded. If it's the magnet type clean it up and space it with a post it note and they'll often run, the more efficient system now providing a much stronger spark which could have been causing a miss. Some are gear driven, and get can get messed up too, either dirty or worn teeth, missing teeth, or simply the gear has slid down the shaft out of position or something. If you're good to there it's probably gotta be the coil or something you missed. Sometimes the magnets become weak, sometimes there's a short somewhere, flywheel spun, maybe a bad ground somewhere, etc.
A few years ago my welding machine caught on fire in the back of my truck, a grounding strap worked loose and it grounded the starter through my oil pressure sensor tube, which of course caught on fire burning up a bunch of wiring. All of which was during a hire in test that i drove through 4 states and loaded up my truck in a couple of days, and i hit a deer on the way there
It can be worse, although it probably doesn't feel that way right now. If you gotta step away from it for a bit absolutely, but then come back later and work it to find the problem when you're fresh, it sucks but it's the best way because you're not only fixing the problem, but you're teaching yourself about your machine, and when problems come up in the future you'll know how to fix at least part of it before you even start.
Finding a spec sheet or owners manual for the exact engine (not the chipper ones but get everything you can find there too) are often a great help too, it'll often have everything from torque specs to gasket requirements, electrical system and wiring diagrams, and info about ignition systems. Sometimes a wrong part is from a different model year or something but their computers don't know it, or you may have to install a different system entirely so you can get parts and do it cheaper/easier than the original system. Often a video can be found to watch too, and even if it's not your exact engine or anything most engines are pretty similar so you can still learn about them more and go back again til you win, using your new tricks to check stuff better. I usually hate it but there's no better way to do it, not to mention the fact i couldn't afford all the junk i have laying around if i had to pay someone to fix it for me
