Sound advice as mine was converted by a previous owner and ade a bit of a bodge of cutting the holes and managed to put the windows in upside down and one the wrong sides, which put the drain holes at the top - spent two days swapping them over and trying to get them to seal up properly. Does not look very nice on the inside either.
Will likely be cheeper as well as new windows = £££££
You may need to take the windows out of a second hand set of panels, and get thm repainted, and then refi, but IT Is EASIER THAT TRYING TO CUT PERFECT HOLES IN A FLIMSELY BIT OF ALLUMINUM.