Is it safe to take a liquid paraffin + milk of magnesia laxative syrup instead of this Ispaghula 3.5gm + Lactitol 10gm Syrup for stomach pain and constipation?

These two are not the same composition, so they aren't like-for-like substitutes. This syrup combines a bulk-forming fibre (ispaghula) with an osmotic laxative (lactitol), whereas a liquid paraffin + milk of magnesia syrup works by lubricating the stool and drawing water into the bowel, and each has its own cautions (for example, prolonged paraffin use, or magnesium in kidney disease). Since you also have stomach pain, please ask your doctor or pharmacist which one suits you rather than swapping on your own.