The rod would be made from a casting, and in general, welding on a casting is a bad idea. The weld can repair the crack but heat makes the surrounding area go through a "heat treatment" that makes the material brittle and more likely to break.
You really, really do not want the rod to break. When it breaks it will be at high RPM and under heavy load. Pieces will go everywhere and get hammered into everything. The entire engine can be instantly reduced to pile of junk and you may not be able to salvage anything from the entire engine/transmission.
A new rod is cheap compared to what you are risking.
I am curious: I have never seen a connecting rod with a crack that was still operating. I have also never seen a crack at the top. I assume you mean the top as being where the wrist pin (piston end) bearing is located. How did you manage to crack it? During operation, or when you were changing the bearing?