The speaker are an odd FORD size, you can just buy or make templates to use round speakers, 6" if I remember right.
My dad has just done the rears on his mk1, used some FLI speakers, sounds really good to be honest, had pioneers in it when we got it, but they were not up to much, and the numpty had had put the three ways in the rear and the two ways in the front, so soon swapped around.
tabetha