Re fuel flow meter
Have you checked Yamaha's website to see if a fuel flow meter is available for your size motor? I have them for my motors ( built into gauges) I would think they would have something for your engine.
Addition here, I just checked their website and apparently they do offer a fuel flow gauge in your horsepower range, for me this would be my preference as these are pretty much plug and play, simple installation and very reliable, both in terms of function and resistance to water/fogging.
I have had no problems with the factory gauges on my boat. Hope that helps.