Window sill repair and or replacement in hillsborough, christchurch

We have a window with a cat flap installed in window. This is not in use and never will be but has caused this window sill to swell. Made from MDF we believe. In the bathroom the same is happening but not as bad yet. Varying other qualities throughout the house. We have just bought the house so dont have the ability to do a massive refit of windows etc. we are potentially looking at retro fit double glazing but they dont do the window sills. Looking for the best options and opinions on what do do next as we dont want these to get any worse and damage the house. Due date : Before Wed, 18th Dec Location: Hillsborough, Christchurch Thanks