Zero maintenance flashing / covers for windows & doors are available. made with either powder-coated aluminum or vinyl.
No mess no fuss. I’ve seen them available at home depot but if it’s a classic window that you’ve got I’d recommend having a window specialist put it on for a professional finish.

If the frame is in good condition you will be able to replace the sill, the old timber sill will come out after removing all the old putty and paint and cement, a bit of care is needed and some thin tools to get into the space between the brickwork and the old sill, study where the fixings might be on the old sill and take your time don’t worry about the mess that will be made on the old brickwork etc. as this can be easily made good, you might find that the frame is screwed into the old sill but the screws can be removed or cut off with a hacksaw blade.