There are different designs of lithium Ion batteries that are packaged in an 18650 package. You want cells that can dump a lot of amps, such as ones designed for use in power tools for example. I have some LG cells designed to pump out ten amps each steady for a time. They can peak close to double that if I recall directly. I have others also from LG that are for a maximum 3 Amp draw, and can be damaged if exceeding something like five or six amps. You can look up the specs of your cell usually if the printing is still on the side. That will give you some idea of how many you would need to parallel to safely reach the peak voltage that you require. If you can couple it with an ultracapacitor bank of Maxwell's boost caps, you may be able to reduce your max amperage for a function like engine starting. One other consideration is that charging Lithium Ion batteries below the 0 degrees can damage them, so if you are in a place that gets a winter climate, or are planning on using your bike in one, that may be a consideration. Auto companies generally use battery heaters to overcome this for larger battery packs, but such a strategy is likely not easy for a small motorcycle or even automobile starting battery.
