You can get a 35mm kit for using a standard 35m film and also mask kits for 16 and 24 frames from a 120 film,I currently have a 16 frame kit installed in my VA MII Rolleicord, they come in leather cases and the type I use requires a separate frame counter that you swap over, I have seen some that consist of just masks but I don't know what model they are for. Here's the same kit I use on Youtube (not my video). Sorry they wouldn't let me post link because I am new around here.
Just put Rolleicor 16 into Youtube search and you will get there.
You must be careful when buying the kits, make sure that it works on your model and that all the parts are there,so many have missing parts or are a mix up.