Google and 30 seconds is all it took.

http://littlewonder.com/12_engine_se...2_pn_385_3.pdf

It's an echo.

I picked up 6 proclaimed dead Echo's from 3 different shops around here.
21, 23.6 and a 25. All of these are basicly the same engine.

I say play with this one and pick up some others too.
Prop hubs and mufflers will all interchange.

These are nice motors with good power.
There are some porting ideas in this forum that will help out a lot.

The best way to go swimming is jump in.
There will always be someone here to give you a hand when needed.

As far as Sq. Inches, what does the manufactor claim?

Have Fun