OK. The difference between the models is this. The MEP-803A makes 60 hertz electricity. Just like what comes out of your wall. The MEP-813A makes 400 hertz. The volts are the same between the two models, just the hertz is different. Why? Well, for certain applications, 400 hertz is preferred, because the time between the sine waves is shorter. Things like Radar, operate better, more effectively. There is less lag time between the sine waves. Most large aircraft use 400 hertz. I spent almost 20 years in HAWK and PATRIOT Air Defense Units, and the entire systems uses only 400 hertz. NOTHING in the system uses 60 hertz.
Now, are you asking what components are different between the sets? There are more then a few. But can you tell the difference between the models? No, without the data plates, its very hard. Most folks would never see the difference. If you have worked with the the 803A, you might notice that there is no 120 volt outlets on the 813A. Or if you opened the control panel and looked at the Hertz meter. Its mid range is 400, its not the same, but easily overlooked. If you open the control panel, the AC volt regulator is marked 400 hertz. S14 is different also. And naturally, the Main Generator, and the control panel wire harness. The best way to tell what is not the same is to check the UOC in the parts manual. There are more then what I have listed here. Not a lot, but enough to make a difference. So you can not just replace the main gen and go on with life.