Intramolecular hydrazide condensation Say I have acetohydrazide ($\ce{CH3CONHNH2}$).

Is it possible, that when I add an acid, an intramolecular condensation reaction occurs, forming the cyclic compound 3-methyl-1⁠H-diazirine:



Furthermore, in general, the mechanism of such a reaction is nucleophilic substitution, right?
Your reaction product is a double bond isomer of a diazirine. 

Halogene-substituted derivatives of the latter are typically prepared via oxidation of amidines with hypochlorites (Graham oxidation).

Heating your hydrazide in an aqueous solution of a mineral acid will only turn it to the corresponding carboxylic acid.