People run all sorts but generally Nosram, LRP and Novak GTB are the most popular systems. If you are just starting out I would recommend a Nosram or LRP 2007 Spec Speedo with a 6.5 motor. The reason I have suggested one of these speedos (they are essentially the same) rather than a Novak GTB is that the Novak is a little bigger and you are tight for space in the 501x.
You can also mix and match and a popular combination is a Nosram/LRP speedo with a Novak Motor.
Hope this helps.
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