Let's see.....

Smaller APC prop.
Bigger carb better exhaust.

If this was a chain saw You should have a working rpm of 10,000 rpm.
If it was a blower then around 8,000.

A 16 x 8 APC prop at 10,000 gives around 19 lbs of thrust.
A 17 x 8 APC prop at 10,000 gives around 25 lbs of thrust

Both of these numbers require less than 5 horsepower.
So you should prop to run in these rpm ranges.

I had a Stihl 088 that ran 13,000 rpm. It had a bored out carb, hollowed out muffler and about .025
milled from the base to raise the compression and raise the port height.

It was very fast.[sm=biggrin.gif]