What features of longitudinal waves take the place of crests and roughs found in transverse waves?
Compressions and rarefactions.
At a compression the local pressure is increased and particle vibration is restricted
At a rarefaction the local pressure is reduced and particle vibration is increased.
I don't think there is a strict convention on translating peaks and troughs to compressions and rarefactions when representing longitudinal waves with a transverse graphic but perhaps the following method will make sense to you. Peaks and troughs could represent the local pressure. So a peak would represent a compression and a trough would represent a rarefaction.