Doug there are two tabs (formed by part of the frame) that stick up through the "rear" of the battery holder, the "rear" being the part closest to the horn relay. Just gently 'untwist" them until they are straight enough to allow the horn relay, battery holder and horn assembly to lift off as one piece. Disassemble, being careful to preserve the insulating fish paper, as it will likely be reusable. The horn relay unscrews from the battery holder assy. Be careful of the single (insulated) wire coming out of the horn, it's not uncommon for the wire to break right where it exits the horn. Unsolder all the wires, being careful to write down where they go or simply take closeup photos from different angles. Once the horn is free, grab a D cell battery and touch one end of the battery to the horn frame and the other end of the battery to the wire coming from the horn. If it works, I'd be really surprised. It should at least click each time you make and break electrical contact with the battery. If it doesn't even click, it's probably a goner! Using a 9 volt battery can sometimes free up stuck contacts, if it doesn't make a sound with even the 9 volt battery, try loosening then tightening the adjustment screw (hex-head) on the back of the horn 1/4 turn, then give the battery another try. If you can't get a sound at all out of the horn by this stage, start looking for a new new (used) horn and battery holder assembly.
I would begin by soaking all the rusted parts in Evapo-Rust, available at your local discount store for about $20, soak the parts in a container, cover it with saran wrap to avoid any splashes or evaporation. For the horn and battery assembly (don't submerge the relay), I would look for a plastic container or tin can that would allow you to stick the horn face down into the Evapo-Rust. The rear of the horn will likely be airtight, so by putting the horn into the container face down (open hole down), you may be able to keep the Evapo-Rust from creeping under the diaphragm and filling that area. You will also conserve the liquid rust remover by using an appropriately sized and shaped container. I typically soak mine for a day at a time, then wire brush the rusted area, then soak it again for another day. That horn will need a few days. Evapo-Rust will remove the black color from chemically blackened parts like the motor side frames, so try not to get in on such parts. The engine's frame will need a repaint anyway, so go ahead and soak that part. When done, rinse with water, thoroughly dry, sand and paint.
On your body shell, look at the front screw hole - they almost always break/crack on that engine, sometimes whole chunks will break away - see if yours is worth saving or reconstructing
Your e-unit is the most likely culprit in the motor-not-turning scenario. If you have repaired e-units before, prepare for another repair! If you have not, there's a bit of "knack" required to get it apart for a proper repair. You can try spraying some CRC contact cleaner directly on the drum and contacts while working the drum back and forth. Be careful, some of the contact cleaners are quite flammable and could give you a singed beard or worse
George
