Buck Boost Circuit for Automobile starter circuit
No, that won't help, you still need a certain amount of POWER to start the engine. If the battery is weak it cannot deliver that power. A buck converter can make more current but at lower voltage but that will not start your engine. 

The only option I see is to use super capacitors as you can charge these slowly even from a weak battery and then extract the energy in a short time to crank the engine.