The first step would be to reconstruct the old piece just to gain an appearance of the old model and to be able to measure its dimensions

In my case, I used the soldering Iron and some leftover plastic pieces to glue everything back together. 

Use a very good  caliper or micrometer to measure the dimensions of what it's left of the original piece. 

Use your favorite solid modeling software to create the design and then export it into STL.

Experiment a few times with different fillings and different layer heights according to your design. If the layer height is too low just because you need a higher resolution, this will not work for small printed volumes unless you have a good cooling fan. The extrusion multiplying factor could be too high for some required precision and the temperatures could create oozing or blobbing. 

In my case, I knew it is a high chance of error and I had the idea of printing like 10 of them with different filaments and settings and after that I choose the best one. 

There is always room for error but this does not mean that there is no room for correcting errors. After the print I sanded and filed the external surfaces, trimmed the support material and remade the holes with a good drill bit. A carving knife could come handy in these situations. I know I got mine.