The MAX7456 video overlay IC can generate a sync signal - I've used it as a display driver, with no input at all. It has some modes for detecting sync in the input, too. Of course, it's for NTSC/PAL, not sure what resolution you are using.
As others have pointed out, it won't magically make a weak signal appear. Sounds like you need better gain at the receive station; maybe an amplifier or directional antenna.
-j