Will a Yamaha banshee engine fit on a 1973 Yamaha RD 350 frame with out fabrication?

Yes it will, the banshee engine is just a water cooled version of an RD engine. The bottom ends are identical except for the rotor end of the crankshaft is slightly longer. You will probably want to get the banshee wiring harness also. (Also a RD350LC was built where the LC at the end stood for Liquid Cooled).