Same happened to me. The quick fix was to tie a tight bit of cord across the deck between the chain plate and the front of the tabernacle with a small loop in it roughly in position of the turning block and reattach the block to the loop with a shackle.
Also not being able to easily source a replacement I then bought a Clevis pin from the chandlery, ground out the old pin, cut down the new one to length refitted the U bracket and peened over the end of the pin.
Incidentally they are identical fittings used on the mainsheet blocks on the transom which can be swapped over If necessary to get out of trouble.
The reason it broke it is that despite new jaws my Spinlock jib halyard clutch slips leading to sagging luff tension. I therefore now put the halyard on the winch, tighten it up and leave it there. However, Note to self.. only have one tin of spinach!