I tend to agree with @Chuck Sartor. I have experienced that problem with a pinched wire shorting out on the engine shell. There are multiple places where a wire could be shorting out or not connected. The E-Unit needs to be completely rewired with a new drum and contact "fingers". Those are available from parts dealers that already have the wires attached. The 671 also has a smoke unit and light bulb, those could be causing some of the shorting. Lastly, the pickup rollers, may also have frayed writes. An engine this old should have all its wired replaced. The pick-up rollers use a special flexible wire, again parts dealers can help.
Do the repair in steps, you can bench test the motor along the way. For example, clamp the motor to a block of wood and test it with wires with alligator clips. You can even run it on the track without the e-unit and body.
I am going to email you a document with a step by step guide. Its for a different engine, but the e-unit and other wiring concepts are relatively close.
At the end, when you get the engine running, its a huge sense of accomplishment and will provide you skills to do other engines.
Best, Dave
