While the Mercedes W123 is an older vehicle, a standard hubcap should fit the car's wheels. Plus, if you bought the car used, the rims may not be the original ones anyway, and they could have been replaced many times over. Replacing the rims should be a relatively easy task. All you need to know is the rim size, which is the "R" number at the end of the tire size printed on your tyre's sidewall. Hubcaps are sold in sizes that correspond to that "R" number.
Use a hubcap removal tool and pry off the old hubcaps. If you don't have a hubcap removal tool, a screwdriver or a thin crowbar should work just as well.
Line up the metal retention hoop with the new hubcap. There is a bend in the hoop, and it should align with the valve stem opening on the hubcap. The bend in the hoop should also "point away" from you
Fasten the retention hoop to the hubcap. Depending on the brand of cap, there should be posts or clips the ring should fit around and into.
Push the hubcap into the wheel. Use both hands and firm pressure.
Tug on the hubcap with your hands. It shouldn't budge. If it does, then you need to troubleshoot the assembly. The likely culprit may be the retention ring, and which way the bend is "pointing."
Repeat this installation process on the remaining three wheels.