Hi Steve
There's a similar discussion in Bike Chat, so this is a similar response!
I nearly purchased a bike that had been powder coated. But the thick coating had hidden the frame number. Was it even a genuine Honda if I couldn't see the frame number? And the colour wasn't right. With powder coating its impossible to match the original colour. Even if the you get the right colour, it looks different because Honda used a particular base, top coat and clear coat to get the finish.
With a classic bike you should be making it as original as possible. If you want it to be more powerful, more durable etc than you wouldn't have bought a bike from the 70s. So I would go for the real thing and use paint! Just my view, but when I repaint my bike it will be with paint not powder
No suggestions for suppliers, but I will be tempted to try it myself at home using the two part K2 aerosol paints.
Keith