Acrylic on canvas
In this little image (only 150mm square*) I tried out an acrylic crackle-glaze on the background. It worked pretty well, but the effect is naturally a bit unpredictable — no doubt it's one of those things that experience would help with.
Once set (and crackly) I rubbed sepia etching ink into the cracks and then polished it off the surface, which actually shows up the cracks immediately rather than having to wait for fifty years for accumulated grime to do the job. I suspect oil paint would work just as well for the job, if you don't happen to have any etching ink lying around.
One thing I found is that the canvas surface is a bit yielding when it comes to rubbing in and polishing off the ink — the process would be easier if the original painting was done on board.
* That's about 6" for those unfortunates still stuck with archaic measurement systems from Ye Olde Tymes