I purchased a Casting Kit from Micro-Mark and made molds for the Rivarossi details for future use down the road. Very few of the locomotives had whistles and few tenders had all four steps and grab rails. I made grab rails from .02" steel rod stock that I bought from Maximum Hobby.
The picture below is a Rivarossi Oil Tender with my DCC mods. I added an extra weight and glued the weights to the sides of the shell instead of the floor. The floor space is reserved for a speaker. I mounted the NMRA DCC connector on a 5/8" diameter styrene tube for easy access.
The DCC decoder is attached to the top of the tender shell with double sided tape.
I made the DCC pigtail 4" long so that it would be long enough for easy connection to the locomotive. I cut a 5/8" x 5/16" hole for the wires to pass through the front end of the tender to the locomotive. I started with a smaller hole but the wires would hang up and cause derails. The larger hole makes for an easier install too, the connector will pass through the larger hole. The wiring harness can be made in advanced then simply installed in one piece instead of soldering on a plug and having to hassle with the shell too.
I mounted all of my DCC connectors on my locomotives with the red wire terminal on the lower right and put a dob of red paint on the connectors for easy identification.
This tender had both rear steps missing, those steps are castings that I made with my casting kit. The Rivarossi tender trucks are not the best trucks in the world by far so I replaced them with Bowser 35107 Buckeye trucks with metal frames and brass wheels. DCC requires good uninterrupted power and the Rivarossi trucks leave a lot to be desired.