You can ditch the ECU if you really want on a 603 but IMHO it is better to keep it at least for the injection pump controls. There is a sensor and actuator on the IP that helps regulate the idle, which is particularly useful if you are heavily loaded with ac and pulling power from the alternator.

Engine will definitely work without it, but you will bog down the idle if your accessory load goes up.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech