He thinks that the block is cracked ... what led him to that conclusion?
Cracked blocks are (thankfully) quite rare and are usually caused through frost damage due to little or no antifreeze.
The symptoms that you describe are much more likely to be due to HG failure in my view. I'd check for that first in the usual way before worrying about the block. If it is the block, I'd look for a good secondhand engine as a replacement, since block repairs can be difficult and may not always be successful.
Regards, John.
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