An SG crank has the pilot shaft for the clutch as an integral part of the crank shaft, whereas the standard crank uses a pilot shaft that screws onto the crank.
This is the SG shaft http://www.amainhobbies.com/product_...SG-Crank-Shaft

To use the OS engines you need a two piece engine mount too. The power of the OS engines is insane compared to the stock HPI engines.