Much as I dread tapping into old wiring, I'm going to hook up a voltage regulator to my RT3 to keep the headlamp from blowing per service bulletin M5-084. Would the below regulator work, or is there one that someone can recommend? Noticed lots of hits on this, but didn't see specific recommendations. Thanks in advance! 

yamaha FS1 6V regulator (treatland.tv)

There are many aftermarket regulators out there like that one.  DEET did a test a few years ago and only found two that worked properly.  RT-507 type from a Vespa scooter (3 wires) or the original Yamaha regulator. 

Lots of people buy the aftermarket ones and don't have issues...  but then there are lots of bikes out there without any regulator at all that don't have issues... 

I used an aftermarket one on an 80cc bike and it seemed to work.  That same type did NOT work on a 175.  So I swapped it with the one from the 80cc bike and then I had two that didn't work!  My guess is some of the one-wire ones for mopeds or small bikes can't handle the power of the larger bike lighting systems. 

I look on ebay for used OEM Yamaha regulators.  Sometimes you can get a used one for $10 or less.  Or contact DEET. 

Or gamble with one of the many available like many people do.