Your Saito .91 4-stroke is at the upper end of the recommended power band for this particular kit. A 5-cylinder radial engine would add so much weight to the front end of your Phaeton II that it would likely make the plane almost impossible to balance properly.

A Saito 5-cylinder radial engine weighs 84 ounces, your Saito .91 weighs about 19.5 ounces.

If you built the Phaeton 90 kit, you might be able to try a 3-cylinder radial motor on the nose, but you'd need an airframe at least that heavy (10 to 11lbs. instead of 6 to 6.5lbs.) to have any hope of balancing the plane.

Good luck, and let us know how your Phaeton II turns out!