I'm not sure of all of the details but Mugen makes an adaptor that converts the threaded shaft to an SG. I have never heard of going form an SG to a threaded one other than cutting the tip off.
What are you trying to run the RG in??
Indy RC World 2.0 - Garland, TX
T&T RC Cars - Plano, TX
Umm....an SG is a pilot shaft engine. So I'm not sure what you are trying to do. I have two RG's I want to get rid of, one new (but not complete) and one used (also not complete). They both have pilot shaft (SG)cranks in them.
goto tower hobbies and type in an advanced search for os rg .21 and there will be a whole list of parts for the os rg but not the rg itself with pilot shaft as it has been discontinued and there you will find the different shafts for it i think there is the pilot shaft and then you need to find out if yours can be fitted or changed into a pilot shaft. this should help. ask tower hobbies if this can be done.
and then scout out a second hand pilot shaft rg engine in parts or non complete for cheap price and install it if your engine can be fitted with it just ask tower as you can ask them questions on there stock, parts, etc.