That looks like a casting flaw to me; if it were a bonafide crack, the crack would manifest itself through the whole surface. I don’t see evidence of that; I see what appears like a superficial scratch in the internal surface only. That said, if it is indeed cracked all the way through, and the dirt and grime is masking it, replacing the crankcase would cure the trouble. I happen to have several 2.5 crankcases that I will never use. If you want them, you can have them for the cost of shipping.

edit: I know I have 2 or 3 crankcases, but I can only find one at the moment. Used bare crankcase with no bearings. The image uploader doesn’t work very well, so I used my image host to post these.