I personally would not go with the G90 for the Showtime, you want something like a nice OS91FX or YS110. I find that the G90 has too much vibration compared with other engines like the OS. Also, the transition is not that great too. The engine is not a screamer too. Will throw a big prop but not a screamer like the OS91FX.

I am pretty sure that the G90 will be able to power the plane nicely and NOT marginal but you need to sort out the muffler, the stock one will be an eyesore for such a nice plane. Try out a nice prop like a 12x9 or 13x8 even if you want it for pattern flying.