I replaced the Shimano U-Brake on my 87 Fisher Hoo-koo-E-Koo with this and it worked well after I replaced the pads.
https://www.modernbike.com/diatech-g...-black-u-brake
I can't vouch for your exact situation but your u-brake looks very similar to the one I replaced. I bet a new u-brake would work, but if you go that route, buy from a shop that allows returns just in case.