Regarding using a car battery to start your bike:
- If your system is 12v and the car battery is 12v it is a match.
- Your starter and ignition system will draw the current they need. They will NOT draw more current. If you have a crappy battery then the starter will suck the current leaving very little for the spark. Also when the plug is in the cylinder head and compression hits it, if there is not enough current the compression will kill the spark. (A little like blowing out a candle). By push starting no current will go to the starter and so this option should provide a better spark. But by using a car battery there will be more than enough amps to use for starter AND spark.