SN 650 impeller upgrade

i was looking to get a new impellar for my completely stock 1990 SN SJ, i cant seem to find this pitch "skat trak 12/15 freeride swirl" what is my next best option? ( i know nothing about the relationship between the first and second number) i dont plan on upgrading to a 701 until next season, as my 650 runs perfect and i just started riding this season. i dont want something that is going to bog down my ski. i dont mind buying another impellar when i do convert to a 701.
 
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