Take a car battery or a 9V battery, or a battery out of a power wheels, I dont care. Make a circuit that reduces the battery voltage to 5V to power the arduino or else you're just melting faces left and right. You can either go through the arduino directly to power the fuel pump and injector (might need to amplify the output/use more power (google)) or use the arduino as essentially a damn in the direct wiring to the fuel pump if it needs more power than the arduino can handle. Once you do some basic math (google) to figure out all your voltages and reducing and amping circuits, wire that bitch up.
Getting the injection timing is kind of iffy. You can use crank sensors and you can just make a fuel map which isn't really crazy difficult (google), although the first time will be frustrating, or you can just guess and check and hope you don't blow anything up. Get your manifold ready and made. Make it so you have leeway with the injector and you can throw spacers in if you want to move the injector up and down depending on how the fuel's mixing (google again... sensing a pattern here? ). You're basically home free at this point. The engine does it's own ignition so you just have to work with that unless you want to get real crazy with it but I don't recommend that.
This was a rudimentary explanation but just one to show it is possible with what you have. You'll need a few more sensors than listed here but if you learn how to use an arduino you can make almost anything.