New fertile areas will become available, Siberia and northern Canada for instance. Do you have a serious scientific source that projects the loss of agricultural capacity to the degree you're suggesting?
The land itself has to be fertile that's why the moon-belt is called like that and is currently where most of the wheat (in europe at least) is being produced. This process happened over thousands of years and is not easy to 'move' or 'prepare' land like that.
New regions would struggle to cultivate new land without fertilizers and the current fertilizer sector would struggle to operate (current sector is built around the availability of ammonia as far as I'm aware).
Your best bets would yes: Siberia, Canada. But it would take maybe few millennia before that land could be used for agriculture and it would effectively become the new oil of the world sparking a new wave of conflicts as I mentioned before.
Our current food supply could be destroyed in decades not millennia given enough heatwaves and other natural events.
