Uhhh, true with respect to your average Wal-Mart Weedeater brand hand held unit. NOT True for any of the commercial grade backpack style jobs. One of the best conversions I've seen is a Shidawa 41cc leafblower engine. Turns a 20x8 with a G45 all day, and is a real smooth and quiet engine.

The guy that taught me to fly has been pushing it around in this plane for about 5 years:

http://www.columbiarc.com/MembersA/image3.html

And some of the commercial leafblowers and weedwhackers actually use the Zenoah engines.

Basically, you really need to check any certain one out to see if it's got the power to weight and can physically be converted to fit a plane.