ifft of signal without phase information
This is tricky. The phase information impacts the time domain form very much. For example white noise and a delta impulse have exactly the same amplitude spectrum, the only difference is in the phase.

Without any further assumptions or extra knowledge this can't be done. For example if you know that the signal is stationary, you could just try a random phase. If it's the impulse response of a causal system, you could try a minimum phase.