Batteries and caps are as different as plants and animals, or talking horsepower versus torque. Fundamentally different.
A capacitor big enough to replace a battery is going to be bigger, so not a benefit. There will be drawbacks too, in what it does to your starter motor and that much current hitting any potential resistance in the electrical path. The cap will also leak down in a matter of days.
A cap used to boost the battery's initial discharge will work, no question, and your friend's bike is an example. But it gives you nothing more than you could get out of adding that much of a lithium battery. The lithium battery will give a far better discharge curve and be more efficient in power to weight. Lithium batteries can hold a charge for years, which is one reason they are used in spacecraft. Lithium batteries have downsides which I've written about them a number of times, but if you want a faster start for less weight (in a nearly 600 lb bike?!) then Li-Ion is a better answer.
Before considering whether to add a starting bank of capacitors, research it.
