I'll also vote for the OS .25FX. I have to admit though, I'm a diehard OS fan. I've thought of trying other engines to save money but when I research them, all I hear is a mixed bag. With OS, you almost never....almost.... hear complaints. Be prepared to pay more money. I believe the OS.25FX sells for around $85.00 or so. With this engine, you'll be at the upper end of the model's stated range. I'm stepping out here but I'd think the .25FX is pretty close to the OS 40LA in terms of power. With 4 strokes, I've heard, but haven't experienced, that the OS puts out more power than a Saito.....this was from a clerk in a store. Its just that the Saitos look so darn good! Perhaps a 40 size 4 stroke?