fred, what are your ideas on what you want to do as far as detailing it? This kit is an awesome build, makes a gorgeous plane when finished. Whatever you do, take your time and you'll have a well built, great flying plane that will get lots of attention and comments at the field.
Regardless of what you decide as far as final appearance (scale yellow, military, etc) these are my recommendations:
If you can afford the cost, get a scale landing gear. Ground handling will be improved, and it will forgive a lot on hard landings, not to mention the improved scale appearance.
www.j3gear.com (William Wallace) or
www.robart.com both have very nice scale gear.
Use a good fabric covering. My first choice, and the most work, is Koverall. My close second choice is Solartex, with a clearcoat after covering to fill the fabric a bit and make maintenance (cleaning) easier.
Four-stroke engine. As bluebiker said, the .65 Saito is a perfect match, and whether sideways or inverted, you'll love the sound as it putts by at 1/4 throttle. You might have to lower the tank floor a little to locate the tank to the best level, but there's plenty of room.
Reinforce the cabin formers (the angled pieces that rise from under the tank to the front edge of the wing, the "A" pillars, and the vertical ones that go from cabin floor to wing center section). A little bit of carbon fiber strip the same width of the wood will add substantial strength without much weight, and will save the center section in an inevitable ground loop. (most all Cub models are susceptible to damage here; not much area for support with all the windows) and is not particular to the Sig kit.
The manual says that the opening window will stay shut without help while in flight. Mine didn't, flapped like a sheet in the wind, even at idle. I fashioned a latch that works with the included door latch, and allows the door and window to be opened independently, and does not modify the kit design. I've included a picture.
Beyond that, knock yourself out and have fun. Start a build thread, post lots of pics.