The 603 main chain links the crank, cam and IP. The vacuum pump is driven off the IP timer sprocket. A shorter chain links the crank and oil pump.

You'll bend valves in a 603 if the chain breaks. The engine will sound different when cranking if the cam isn't spinning. You would have noted that in your message so the problem is probably in the IP or timer.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD