Wow - it's beautiful.
If you look at the How To Restore : Speedometers page, you'll see that the speedo is from 1951, and it appears that the entire chassis is a 1951 Model 165. In my opinion, this is the best looking of all the Hummer-series. I'm a little biased, as I had one years ago. If you look at
this page scroll to the bottom, and there is Barbara with my 1951 in the middle of the National Mall - before the U.S. Park Police came and chased us off.
Your engine, however, is a 1958 Hummer, which bolts up and works fine. But we'll have to do some custom wiring. The Hummer does not use an ignition switch - just kick it and it will start. It has a kill button on the cover next to the points. The Model 165 chassis has an ignition-park-lights switch mounted in the gas tank. I would suggest you use this to turn the lights on and off, and forget the ignition function. Just don't park it in a high-crime area.
The Super 10, Pacer, Scat wiring diagram is the closest thing. But since you don't have the Hummer headlight off-low-high switch, you'll have to modify the wiring to use the Model 165 ignition-park-lights switch along with the Model 165 handlebar low-high switch. There are some layout differences - the 1960s Super 10, Pacer, Scats have the horn located in the front fork area, while the Model 165 chassis has the horn on the frame towards the rear of the bike.
We don't know what model your wiring harness was originally intended for, but whatever it was, it's not gonna work right with this custom job. My suggestion is that you just buy some spools of black, red, and green wire, and wire it up by hand. Once you've got it wired up and working correctly, go back and modify the harness so it fits the custom application. Yes, it's double work, but I think you'll save yourself a lot of heartache and head-scratching. A Brother p-touch label maker might be a big help for marking wires. Masking tape and a Sharpie works just fine.
I will look over the wiring diagram this weekend and try to offer some more detailed info. Hopefully, the above will get you started in the right direction.
Dave