Ebay might be your best bet. I dont think you will find anyone still making or stocking parts for 300s these days.
I love 300s and I have built two of them. One has pretty much every bolt-on aftermarket part available to my knowledge and the other is completely stock apart from a 38mm bn carb.
Both are very similar in terms of throttle response and top speed. The carb is the single biggest improvement that you can do to a js300 motor to make it go good and probably the only thing worth doing (You cant beat the look of a nice shiny chrome pipe hanging off the engine tho!).
Hope this helps