Tyres.co.uk has the Michelin X 125 R15, Nankang CX-668 and Toyo 310 in 135/80 R15 as period replacement options for the factory-fitted Michelin X 125 R15. However, please be careful when ordering. Classic cars, especially, may have aftermarket modifications that may be non-standard. Please confirm the wheel, load and speed ratings before placing an order.
The factory tyre pressure placard for the Citroen 2CV6 is located on the inside, lower side of the driver's door, just under the door check strap. The placard has some inflation guidance, but to get the full tyre markings, you need to check the sidewall of the tyre, and the owner's manual should also be referenced as aftermarket changes can affect what is fitted.
Because the Citroen 2CV6 is front-wheel drive, only the front wheels need to be replaced. For safety reasons, when you replace tyres, do so as an axle pair, and when you fit two new tyres, UK advice is to fit them on the rear axle.