First, get a wiring diagram so that you can see what you are doing. When my '75 750F arrived (an abandoned orphan) it had a few electrical problems and I had no wiring diagram. I spent more time than I would have liked following wires around the bike to figure out how they did electrical things at Honda back in the '70's. Now I was able to do this but I have a little more experience with 12 volt electrical circuits than most people on this board (and there are still some peoply here who make me look like a beginner) but it sure would have been a lot easier to see it on paper.
You can read the wiring diagrams just like a roadmap, you just have to learn the symbols. Also check out the interactive wiring diagram that has been mentioned in other posts, it will give you a better understanding of the systems. Most of the circuits on the '75-'76 F are similar to the earlier models with the most notable difference being the lighting,as with front running lights and no headlight switch.
To answer the question you asked about the headlight, you must understand how the headlight circuit works. The starter button is essentially a spdt (single pole double throw) momentary switch. What that means is that when the key is turned on one wire to the switch is turned on to the center or common terminal as it is called. The other terminals are called normally closed - NC, and normally open - NO. Current is provided to the normally closed terminal at all times except when the switch is pushed, this powers the headlight. When the button is pushed current is interrupted to the headlight and the momentary position switches to the normally open terminal and activates the starter solenoid for as long as you press the button.
Someone on this board has mentioned purchasing a SPDT momentary push button switch from a source like Radio Shack and simply replacing the switch. I do not agree with this method because I feel that due to the size of the switch that it will not be able to handle the amount of current a headlight requires, typically about 5 or more amps.
Another approach is to obtain replacement parts from Honda or from a spare switch to repair your switch, maybe this can be done with a start only button on the earlier bikes, but with the double throw switches unlikely. I beleive that these switches are a problem because Honda didn't get it right, too much current for those little contacts in a very small space and they wear out very quickly.
The way I did mine will outlast any Honda switch and probably the bike.
You will need one small momentary push button switch, just a single throw switch, low current is OK, but be sure that it will fit in the space where the starter button is. I got a cheap Radio Shack switch. You will also need a good SPDT automotive lighting relay. You locate the relay in the wiring enclosure at the top of the downtubes. Unplug the headlight wire to the starter button and connect to the NC terminal of the relay. Unplug the switched power to the starter button and connect to the common terminal and also to one side of the coil in the relay. Connect the starter wire to the NO relay terminal. Mount the switch in place of the starter button. You should have 2 wires from the switch. Bring those to the electrical enclosure. Connect one to the remaining relay coil terminal and the other to a good ground.
Sounds much more complicated than it is, really a very simple relay circuit. It takes the heavy headlight current from the switch to the relay. You are looking at a swithc for less than $1 and the relay should be about $6 or less if you get a good deal.
The relay works exactly the same way the original switch did. NC terminal powers the headlight whenever the key is on, NO terminal works the starter solenoid when you push the button. The PB switch you replaced the starter button with activates the relay by energizing the coil which makes the contacts switch.
As far as your no power problem are you sure about the fuses? Get a test light with a large bulb (you want to load the circuit a little bit) ground the clip, turn the key on and check each side of all 3 (I think) fuses. The light should shine equally bright on either side. Do not rely on visual observations to check fuses. Clean the corrosion from the fuse terminals and any other electrical contact. There may be 2 spare fuses in your fuse box whih are marked as "SP". If the fuses are ok check you key switch for voltage out with the test light in the same manner (clip grounded and probe terminals for power).
Good luck
