Can carotid arteries be replaced if they are too blocked for carotid end-arterectomy?

No. If you have a complete blockage of the carotid artery, then no treatment is advised. If blockage is severe, carotid endarterectomy or stenting can be done. The degree of blockage and presence of symptoms determine the need for intervention.
Yes . It depends on the area of blockage. Saphenous vein grafts or bypasses can be helpful. Goretex and dacron grafts can be used. If the blockage is too high, intracranial bypass may be an option in certain settings. At the carotid bifurcation even very bad blockage is usually still suitable for endarterectomy.
Yes, In development. Arteries are being grown in tissue culture, typically using an animal artery, slowly washing away all the cells using detergents and then perfusing cells from intended recipient so as to largely prevent rejection of foreign cells/tissue. However the better course would be to aggressively address the driving factors: smoking, lipoproteins, blood sugar, blood pressure, etc. Which drive the ds.