Can a stroke be congenital?

Sort of. You can have a stroke in the uterus (extremely, extremely rare) or any time after you are born. It is not really 'congenital' per se. What could be considered congenital would be risk factors such as a genetic predisposition to develop aneurysms in your brain or vascular malformations in your brain or bad athersclerotic disease of the arteries (hardening of the arteries).