video - how to convert a Sync-on-green to VGA (seperate sync) signal?

I have a computer (VAXstation 3100/M76), that has a sync-on-green output:

red          --(
green + sync --(
blue         --(

I want to attach that to a standard VGA monitor, which has RGB signals, and H-Sync and V-Sync on separate signals.

I found several circuits that work the other way round, making a sync-on-green signal from separate sync signals, but nothing that would solve my problem.

Is there an easy way of making a sync-on-green to VGA adapter?