it totally depends on the person. You could start body recomp (gain muscle and lose fat) and stay at 49.5kg but your physical measurements might get smaller bc muscle is denser than fat. You're a healthy weight now so you couldn't lose more than 4 or 5 kg at a push w/out being underweight.
45kg would take you to the brink of being underweight, would not recommend that. Building muscle would make your physique look much better rather than becoming more skinny.
Here is a routine that doesn't require weights.
It depends on the age, but 49.5kg is perfectly alright for a young female at 5'1". It's not underweight at all, but a desire lose weight to fit into an XS is concerning.