Sorry- I'll give you some more, athough others will chime in too. If all you need is a healthy spark, a headlamp and a horn, then, IMO, the stock alternator is more than enough.
The spark is delivered by the coils which are a transformer, of course. More voltage to the coils translates to more voltage (and 'better' spark) at the plug (within reason, of course). A properly charging system will provide the maximum safe and steady voltage to the coils necessary for a proper spark.
You cannot get around Ohm's law- V=IR. If you wire in a bunch of silly accessories, they draw power. As they draw power, the available amperage from the alternator drops. Consider R to be all the resistive power-consuming parts like a light or a horn. I is fixed by your alternator, and V drops from the max allowed by the voltage regulator down to whatever it happens to be. That is- as R goes up, V goes down, as power consumption goes up- the available volts go down.
The battery acts like a power buffer (providing I for a short time), but it is limited. After that- the battery runs down, the alternator is still putting out max amps but if they aren't enough then voltage to everything drops. At that point, it's a losing battle. The battery HAS to have above 12.6 V (~14 V for new gel batteries) to recharge. 11.5 V at 100 amps wont recharge your battery. 14.2 V at 0.2 A will. So- you need both Volts and Amps to get the job done. I need lots of both, since I want to run a cooler. You probably don't since you have more modest requirements. Your bike will corner better, as well.