The one I have with my airfix kit dosn't fit and is also chipped. Does anyone have a spare by any chance?

if it is too small you could try smash molding a replacement from some clear plastic.

Just secure the canopy on a dowel, heat the plastic sheet till soft and pull it over
the old canopy. If it is too big this may only make you problem worse, but mind you
canopies made this way are more flexible and easier to trim
so you might just be able to manhandle it into position.

Also it will be thinner and more transparent and lees distortion,
prob not a good thing on and airfix kit
Works best with simple shapes too. Like say a p-47 bubble top.
Hard to do with something like a lanc cockpit canopy.

P61 has a lot of glass. What does the kit part look like?

might take several goes to get one that looks ok, don't be disheartened if
the first few break or are all crumpled.

Good luck finding or manufacturing a replacment