As best as I can tell, the Satio FG-11-4c engine makes about one HP, similar to a .40 glow and should work well on a 1/6 scale Sig Cub. I would not worry about it being overpowered. It won't be overpowered by all that much, you have a throttle, and a little extra available smash might just help get you out of a jam sometime. The Saito's all-up weight of 25.4 ounces is about 8 to 10 ounces heavier than a typical .40 glow engine with muffler. This might make the plane nose heavy, depending on the battery and ignition location. The electric setup with batteries may also weigh more than a glow engine configuration, but the batteries can be moved around to adjust the balance much easier than adding lead to the tail for a gas engine. As for realism, I've never seen a J-3 Cub fly by going whir-whir. It has always been putt-putt-popity-putt-putt.
As a reference, my Carl Goldberg Anniversary Cub weighs about 7.5 pounds and has been flying very scale like with an Enya .53-4C glow engine (0.8HP) for almost 30 years now (obviously not continuously). It flies about 10 to 12 minutes on 6 oz of glow fuel at half throttle. Glow fuel is still an option at this size. Glow is messy and batteries are not. Gas is only very slightly messy once the engine is broken in and switched to synthetic oil mixes. It is easy to refuel a gas/glow Cub. If a suitable hatch location is not available, the wing might need to be removed to change batteries. Each has its pros and cons. All things considered; you would probably be happy with any of these power options, so get what you like and then fly it like you stole it.
The Saito data I found from Chief Aircraft is:
- Fuel Type: Gas
- Cylinders: Single
- Displacement: 0.66 cu. in. (10.88cc)
- Type: 4-stroke airplane engine
- Bore: 1.06 in (27.0mm)
- Stroke: 0.75 in (19.0mm)
- Cylinder Type: Ringed AAC
- Plug Type: 1/4-32
- Total Weight: 25.4 oz (720 g)
- Engine (Only) Weight: 16.2 oz (460 g)
- Muffler Weight: 1.80 oz (50 g)
- Ignition Weight: 3.90 oz (110 g)
- Crankshaft Threads: M7 x 1
- Muffler Threads: M12 x 1
- Benchmark Prop: APC 13 x 7 @ 8800 rpm
- Prop Range: 12 x 8 - 13 x 8
- RPM Range: 2,000-9,500
- Oil Type & Content: 20:1
- Muffler Type: Cast
- Crank Type: Saito Walbro