i have had two cases where the key couldn't start the car, one was when the ignition lock developed an internal fault which meant it would crank the engine but not actually connect to the ignition system. the other was when i first installed my aftermarket ecu, i initially used the power to the heater blower as the source for the ecu. of course the heater blower along with various other things is cutout when you put the key to start position.

for this kind of non start case most of the tests you need to do apply equally to modified and non modified cars. should be something in the haynes manual to work through - cranking or not, do you have a spark, is the fuel pump running etc.