FWIW, I made my own composite adapter for PET video output too, but it only works with non-CRTC PETs, on CRTC based boards I've found that the syncs are very similar to those of PC's MDA and EGA monitors, but I had to invert one TTL sync signal (horizontal, afair) and the monitor worked very well with a "FAT-40" video signal. I made a little board with both adapters combined (it only needs a single LS02 and some passives for both, but I added some jumpers to select what the circuit needs to do).

Frank IZ8DWF