I built the Sig 1/5 Cub and flew it this past summer. It flew beautiful with an inverted Saito 65 with an onboard glow, much more power than necessary for me. I have since pulled off the .65 and have put on a Saito .50 to bring it to a more realistic power vs. plane ratio for myself. I haven't flown it as of yet, but the Spring will be here soon! I spoke with Gary from Sig( I think that was his name, maybe Glenn???) and he had told me that he used a Saito .45 in his and it produced a very realistic flight performance that he was happy with.
Granted, you can always throttle back with the larger engine, but the Cub seems to fly on minimal power to begin with, similar to the real plane. The Saito .65 can utilize the factory Saito right angle exhaust adapter made for it, which will hide the exhuast in the cowl. If you also use the flex exhuast pipe, you can leave just enough pipe extruding through the bottom of the cowl to give it an original appearence.
Also, I cut out in between the exhuast pipes on the dummy engine to aid in the cooling of the engine. On the oil cooler screen on the bottom front of the cowl, I cut out the center and glued in a piece of black window screen material that also helps the flow of air through the cowl and looks pretty real from a distance also. I had no problems with engine operation in this plane, it just had waaay too much power for what I need with the .65 in it. The .65 is an awsome engine and a dependable engine although slightly on the heavy side.
By the way Ken Hodges, if you read this (and I'm sure you will) I picked up a set of the Robart 1/5 Cub landing gear, got it for a price I just couldn't resist! How many flights do you have on that grasshopper Ken??