Just something like this: http://www.ebay.co.uk/itm/221262872137
Looking at SCART wiring diagrams it should be wired up correctly. I don't see any pins for Y Pb or Pr so I'm guessing it would just be wired to R G B anyway and maybe wiring to composite would be the same as having composite sync? I don't really know though. If it wasn't you could just open up the end and put them where you need them, then get 4 RCA-BNC connectors for the R G B S ends. And of course add resistors where needed if they weren't already there.
Looking at SCART wiring diagrams it should be wired up correctly. I don't see any pins for Y Pb or Pr so I'm guessing it would just be wired to R G B anyway and maybe wiring to composite would be the same as having composite sync? I don't really know though. If it wasn't you could just open up the end and put them where you need them, then get 4 RCA-BNC connectors for the R G B S ends. And of course add resistors where needed if they weren't already there.