I had to do this to to mine as well. You will have to get an oversized threaded insert put in there. Any machine shop can make you one up. Not sure if it’s legal but I’m not ditching the block because of it.
Yes, you can use a Keensert. There are other brands as well. Choose one "slightly" bigger than the heli-coil od, since there's not much room to work with on the upper right hand header bolt before it breaks through the fin/deck of the block.
I've got a fixture for the mill that I do this in, but you can free-hand it if you're careful and take your time.
I charge $15 to install a thread insert (includes the insert) if you get in a bind and feel you can't handle it yourself.