Electrical issue
 

My chinese gy6 suddenly had no spark, so I swapped the cdi and ignition coil, which seemed to fix the problem for a few hours when there was no spark again. What could cause this?

Check for bad connections would be the first that comes to mind. Loose boot cap on the coil wire loose connections on the coil itself, bad or loose ground wire from motor to frame etc.. Also CDI may be overheating to close to something giving off a lot of heat, Bad Pickup coil(which can be tested). could also be you got a bad coil and or CDI don't forget all this stuff is made in china and some or better quality than others.
could also be a bad stator as well(that can be tested).

Could just be a bad plug internal crack.
Lefty

I have new ignition coil, cdi, spark plug and ignition switch, turns out all were faulty! New ignition switch has internal problem, spark works fine when it's disconnected so there's a short in there. Now all I have to do to be back on the road is clean the carb. (probably should have done that 6 months ago...)