Most modelers don't realize just how much improvement can be achieved with a simple propeller change. You want to prop this engine to reach at least 12,000 rpm on the ground so that it is closer to it's peak power. Going to a lower pitch larger diameter will make it pull harder on the ground so it will accelerate better to flying speed.

I agree with John, Ken, Grey Beard, and others. Try the 11-4.

Bigger diameter tires too.

If none of that works, hand launching always does.