That block was cracked back in the 80's when I owned it. I had a guy who specialized in welding cast iron come in from out of town to try his best to repair the crack. As he welded the original crack with nickel rod he'd peen the surrounding area to try to keep it from cracking again as it cooled. Japanese cast being what it is, there was no joy, it just kept cracking.
I repaired the cracks I found by grinding them out and filling them in with JB Weld because I didn't want to pull the engine out again. It did seem to hold for all these years, but the engine probably had no more than 20 miles on it. It seems there are still some hairline cracks that have opened up. I wouldn't have reinstalled it without swaping the guts into a new block, but that's water over the dam now. You might want to try doing what I did back in the eighties and just grind the cracks out and JB Weld them. It did hold a pressure test back then after the first repair.