will they start mining the chain from the genesis block?
As far as I know bitcoin core never automatically started mining, you had to manually start mining new blocks.
you have
CVE-2010-5139 when block 74638 contained a transaction that created over 184 billion bitcoins for two different addresses. after this block chain had to be forked and all old nodes need to update them code. so bitcoin 0.1.0 can't work then.
That block no longer exists so the node should technically be able to sync since it follows the longest chain. Also mining a new block with the bug is too expensive an attack for someone to pull off today just to affect the 0.1.0 version.