I just purchased an 81 Datsun 280zx that wouldn't start. I combed through lots of forums to figure out a starting/ignition issue. Here are some things I discovered to help narrow down the problem. Maybe this will help others.
Car would only crank while in neutral. The park/neutral switch is possibly bad. I moved on to the next item as I could get engine to crank in neutral, but still no start.
I then identified that I wasn't getting a spark from the ignition coil. I noticed that the coil positive/negative connections were incorrect which I fixed, but that didn't help. I bypassed the ignition transistor and grounded out the coil on the negative terminal while the while key was in the start position. This gave off no spark, so I replaced the coil. A new coil was able to provide spark using the same test method above.
The next thing I discovered was not receiving proper voltage drop/ground from transistor to coil negative while cranking. I checked the ECU for any visible blown resistors and found none. I decided to replace the old power transistor with a PRW-2 transistor. The link with additional details can be found here, https://www.zdriver.com/forums/280zx-s130-forums-77/1983-280zxt-ignition-transistor-pack-upgrade-42641/
Once the transistor was replaced I was able to get a spark from the new coil during engine crank. I replaced the aging spark plugs and spark plug wires. Things got much better after that.
Here's a YT video on some of the steps performed, enjoy: