Are you sure it reads cc's after the 20?

The most common trainer has a 0.40 cubic inch glow engine (it uses nitro-methane as fuel).
Engines of 40 cc are fueled by gasoline and are big and heavy.

This site has plans for a 0.20~0.25 cubic inch and step by step with many pictures; however, it is in Spanish:

http://picayzumba.com/content-cat-6.html

I believe you will be able to bild it if you use a on-line translator:

http://www.altavista.com/