Generator repairs can be tough sometimes. Lots of electrical components to take a look at. I'm not sure at what point on the generator you did your testing so I will start at the beginning.
You are going to need a Multimeter to do the testing. REMEMBER you will be working with 120V and 240V power! If you are not familiar with basic electrical principals and the precautions required to work with electricity stop now and take the generator to a service center.
First take a reading directly off of the stator. There should be two sides to the stator. Each side should be outputting 120V. The generator ties both sides of the stator together to give you your 240v output. If you get good 120V readings off both sides of the stator then it should be fine. If not you will need to disassemble the stator assembly and look at the internal components. Inside you are going to see basically the workings of an electric motor. Check the stator and the rotor for any obvious signs of damage or overheating. There are two rectifiers and two varistors that could be bad.
If the Stator tests OK then you need to look at the wiring between the panel and the generator. Check the wiring harnesses for continuity and obvious signs of heat damage or electrical shorts. Make sure all of the connections are tight and not damaged.
I hope this helps. It is going to take a lot of troubleshooting to solve this one.
Thank you very much for your advise,and yes I'm familier wit tesing heavy or high voltage electrical components ,but not very much experience working on a generator before.however your advise is very much helpful to me.I've started at the panel and then I did I ended up at the stator,I did not disassemble the stator section,but according to the circuit diagram the two parts you've mentioned are the componenets that I suspect are shot.