Here is a quote from a guy here on RCU that you may be interested in.
"Saito 1.20 4s vs. G-23
By: Mike Cunningham - 1 Kit Tip Posted | 1 Product Rated
Posted: 10/12/02
I have had both engines installed in mine and the only real difference is that the landing speed is slightly higher with the G-23 due to the extra weight. It is a near crawl with the 1.20 though and is still fairly slow when carrying the extra weight of the gasser. The plane requires some lead in the tail to balance with the G-23. It is hard to say which is the better choice."

This was found on RCU by first looking for Spacewalker in the sell pages then following the link to the review section.

If your going to fly it as a sports plane then the S3003 will not do.

Try the Hitec 475. They should do nicely.

If you were just going to go out an float around and drive like, well like somebodys grandma then the S3003 would work.

The 38cc engine is going to be pretty heavy.m A Saito 125 only 28 ounces. Your looking at close to 3 pounds or 48 ounces.

Start off with the battery, servos and reciever in the tail section and shortining the noce by an inch or so.