There are a few ways of achieving this. The easiest and least invasive way is to command control the variable voltage supplied to the track block with a PowerMaster and requisite command control system. This doesn't require opening any locomotives and undoing any of their original internals and whatnot. Thus, it is easily more economical since once done to the layout, there is no per locomotive costs. However, it doesn't unlock any features for which the locomotive is not currently equipped.
If you want to add it per locomotive, it isn't going to be inexpensive. First, you need to get the ERR board. As your postwar locomotives have AC open frame motors, you'll want the ERR AC/DC Commander. For sound, you need to select the appropriate accompanying ERR RailSounds Commander kit. You'll also need a compatible Lionel R2LC board. Then there are the miscellaneous items, such as wiring looms and connectors, any tethering connections, electro-coupler upgrades and on and on and now you've potentially spent more on the upgrades than the locomotive itself and now you have to put it all together. It can be done, but do not forget that acquiring an equivalent or better TMCC equipped locomotive can frequently be more cost effective. Additionally, any potentially collectible items of the postwar era, well, they are only original once...
