I have learned to never used a cracked anything. the new stresses caused by retorquing, etc can cause the crack to expand, and that usually happens when you are 100 miles from home, worst type of weather, etc.
Do not chance it. Welding can work, but the chances of the weld cracking, or new cracks developing are increased due to the heat introduced by welding. Cast things like blocks and heads usually do not show good results from welding. Best thing to do is find a good one in a wrecking yard.