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Standard units are 10cm / 4" round with 4-ohm impedence. A standard 10cm speaker should be a straight swap. Many people have upgraded to a 5.25" round speaker without any modification needed. The clearance has been reported as 75mm / 3".
Going with some speakers that are too large for the opening or too deep for the door may require placing the speakers over the door card, surface mounted.
The original speaker cones usually rot away although the speakers continue to operate (and don't sound particularly distorted despite having half the cone missing)
The rear speakers, residing in small boxes behind the seats, are 130 mm / 5.25" round. The box can be cut to accommodate speakers in the range of 6.5" round, or 6x9 oval.