RE: Engine and Pipe Decision.
The Ofna Clutch Bell with part number 13999 is meant to be put on a buggy and has 13T. You can also get a 12T bell with part number 13998. I would think that they can be used on a OS RG engine especially if you use the Ofna flywheel and clutch shoes. Remember that the OS RG can be had with or without a pilotshaft.