they are the same. The parts breakdown on the X-12 sheet (the one that comes with the Mugen) has a small section called technical data. This section gives horsepower, rpm, ports, bore,etc..... The two engines listed are NS12S3 and NS12S5. The parts breakdown (exploded drawing of the engine) shows a part number for a 5 port p/s. To the best of my knowledge there isn't a 5 port Mugen. It is the same engine. If you think it runs better, then get a Mugen.

Off the mugen sheet:

Part #
NOV08606 piston and sleeve
NOV07601 conrod
NOV15616 11.5mm crankshaft
Off the Novarossi sheet

Part#
08606 piston and sleeve
07601 conrod
15616 11.5mm crankshaft