I guess the first decision is if you want a Rat Bike, concourse or something in between.
If it were mine then I would start with getting the engine working; if that's Donald Ducked then the rest of the machine becomes a big foot powered scooter.
Depending on what you find will guide what needs to be done.
Start with the basics - does it turn over, what compression does it have? Drain the oil and see what condition it is in and does anything come out with it.
Replace the spark plugs, check valve clearances, contact breaker condition, check ignition timing, cam chain adjustment, clean or replace the air filter.
Strip the carbs, clean and assess what rectification is required. If badly gummed up then you may need to ultrasonic clean them.
I would then move onto the electrics. Have a wiring diagram to hand and systematically work through the systems; check off on the diagram which bits you have done. Look out for any signs of a butchered loom which should ring alarm bells. You'll probably realise quickly if the loom is serviceable or if it would be better to start with a fresh one. Where possible clean and check the components attached to the loom.
Engine OK, loom and electrics OK? If you have a header tank then now might be the time to see if any life can be induced into the motor.
Check the inside of the fuel tank for signs of corrosion and take appropriate action.
Assuming all of the above has gone well then it's time to turn your attention to the cycle parts and work through them.
And most importantly, always have a note book to hand so that you can record the 'to do' list and those random thoughts that come to mind as you work.
