Syrup with magnesium carbonate

A physician specialist in internal Medicine prescribes the following magistal preparation: R/Citric acid 31g Magnesium Subcarb Lev 18g Aqua 202g Sirupus simplex Conservans 47g We have no information about the posology, nor the indication (phosphate binder?). Mg Subcarbonate Lev. Cannot be ordered (anymore). Can this be replaced by the heavy variant? Or can magnesium citrate (anhydr) be used here, and if so, what quantity?