ORIGINAL: HighPlains
Your engine selection is going to severely limit the speed you can achieve. It will have to be prop'ed to turn 16,000 in the air to reach it’s peak power, which is only around 1.6 hp. With a stock Quickie 500 design it will top out at around 120 mph, but with a slightly smaller cleaner airframe you might reach 140 if the drag is low enough to turn a slightly over square prop.
What engine would you recommend, I only used the AX as an example? I need to keep my choices to locally available engines (Australia) that won't burn a hole in my pocket.