Probably not the recommended advice but I'd be tempted to disassemble the whole mechanism, clean and regrease etc and then bend that screw carefully straight again and any sign of weakness/damage to the metal of the screw replace it. I think both my Paris Explorer and Muddyfox folding bikes have similar latching mechanisms. If you need to replace the screw perhaps spend some time measuring it accurately and then see if you local hardware shop can supply one if its anything non-standard. I keep a jar of old screws that hasn't failed to provide me with any screws I need even if I have to cut them down occasionally. You can zinc plate/galvanish on the cheap if the screw you find/buy is likely to rust.