The dB(A) levels are derived from the power spectrum, so if you can calculate the power spectrum then you do not need to back-calculate, you already have the power spectrum.
If, however, you have not the power spectrum (contrary to the statement in OP) but only the dB(A) spectrum, then you can obtain the power spectrum by subtracting the relative dB(A) weighting levels from dB(A) spectrum.
If you have only the broadband dB(A) reading, you cannot go backwards, as the integration (dB(A) filter) has already be done.
Edit: a related question can be found here: https://electronics.stackexchange.com/questions/173989/convert-db-a-to-db-spl, but contrary to that question the OP here assumes to have the power spectrum available.