Welcome to the forums, newcomer!
Due to your form factor, the only option for a replacement PSU is one with the same wattage and the PSU or the build never came with a higher wattage unit. Unfortunately you can't invest in a PSU either since your chassis will prevent you from dropping in an aftermarket PSU.
Long story short, just to replace the PSU, you're looking at replacing the cassis too prior to you dropping in a discrete GPU, I mean that's the only reason why you're looking for a higher wattage PSU. Also, prebuilts don't have their front panel connectors labelled so hooking them up will be troublesome.