Welcome to iboats...........
There probably is not a "standard" diagram because many boats are set up and equipped at the dealer. Wiring for different makes of outboards, different fish finders, gauges - all slightly different.
What specific system are you trying to get to work?
Lighting is fairly straight forward and so is a horn. Tachometers vary so do feul gauges and trim gauges too
The current wires you have might be fine but might have corrosion inside them. Many times people have been able to replace the first 12-15" of wire to defeat the corroding.
Rules for wiring - one wire at a time...... solder where possible or twist together and tape - do not use "scotch locks" and sometimes sanding all connections works too.
