Broken vertical casement window sash lock

I have casement Marvin windows installed 1994 and need to repair or replace a broken sash lock. There is an upper and lower lock connected by a wire rod. The upper lock is fine but the lower one is broken. The plastic piece which slides up to engage the lower sash keeper is broken and will not stay attached to the connecting rod. If I can't find a replacement plastic part I am fine replacing the entire mechanism, upper and lower locks. Thank you.
J Lu