so my question is: is there a replacement ic, which has the same pin layout and works with a power suply of 1,5V
or should i use 4,5 volt + a resistor.
There is always some voltage drop through the H-bridge. With the L293D, it is about 1.2v. I don't know of any replacements with a lower dropout. If the voltage is still too high, you can use a lower throttle setting via setSpeed(). The PWM will reduce the effective voltage being sent to the motor.