You can use the BW35 (long tail version preferably) from a 140 in a 120-130 allright. You will need an oil cooler then but that will extend the life of your transmission. A separate oil cooler can be positioned in front of the radiator. The only other necessary modification is bending the dipstick tube somewhat. Ater that you’ll notice the 140 BW35 dipstick will not fit into the tube anymore. However the dipstick from your 120 BW35 will fit because it is partly flexible. It wil only need minor recalibration.

On the picture below there's a 140 BW35 in the foreground and a 120 BW35 in the background. Note the different mountings for the dipsticktube.