Still a beginner here, I had a roll over and my left front motor was doing the twitching thing.  Found the same problem on youtube where it said the engine had to be replaced.  Ok, so I replaced the motor and it is still Twitching. I now have a spare motor.  What is my next step?

Next step is the ESC ( motor controler) for that engine. Mine was the reverse. Thought it was an ESC issue and ended up being the motor. They work in tandem. Sometime had to diagnose which it is so sometimes better to replace both.  Motor diagnostics... Do they star and stop at same time. If possible check the RPM at idle and full speed (without props). Are they within 50 RPM's of each other. Then turn the motors one at a time by hand and feal the resistance, do they all have the same feal and lastly listen for any bearing noise and check motor by pulling up and pushing down dealing for any abnormal free play. Shows up different that the others.  Have to be careful too as a bad ESC can give false RPM data too. How to tell if ESC is bad.... If you replace the motor and the same issue is with both the new and the old motor... Suspect the ESC!

I just went though that with mine. I had to replace motor and ESC. It involves soldering and a steady hand. BTW.. How do you measure RPM on each motor?

I use a  Laser Light RPM tach. You can get them on eBay fro under $20.00 buck with the shipping. Good investment for good tests of the engines/ESC's.