I honestly thought it was easier.
In fact I recycle the rosin that I recovered from cutlery that I melted, on the other hand I would not know where to buy new rosin.
This rosin is not homogeneous and with boiling water it becomes only creamy and it is not easy to pour it into the hole of the handle.
However, I tried another method to get the job done.
I used 7 mm diameter sticks of hot glue. I held the handle in a vise and heated it with the heat gun to the melting point of the glue, then pushed in a couple of sticks, until the handle was full.
I inserted the blade and waited for it to cool down. I removed the glue that had come out with a hard plastic spatula and cleaned the remaining traces with ethyl alcohol. For the moment the work seems to have been successful. I don't know if problems will arise in the future, but I don't think so.