If you end up getting an ST G51, I suggest a muffler from Randy Smith and a new ring from Frank Bowman. I am sure with that you'll be able to tune the engine satisfactory as many have done it.
Have you considered the Enya SS50? It is lighter, stronger and it will last for a long long time. Also the engine does not need to get it reworked or have parts replaced, works amazingly well out of the box.
Enya SS50 with a cheap APC 12.25x3.75 prop do a very strong 4-2-4
I haven't tried but it feels strong enough to turn a 13x4 or APC 13x4w, or better yet a 12x4 3 blade carbon fiber prop.
Here is a video of one of the first flights my friend Rogerio made using the Enya SS50, APC 12.25x3.75, 10/22 fuel, Enya 3 plug, AirFilter on the ventruri. At that time the Fuel tank was not fine tuned, later he lower the thank an got it perfect. I have to make another video to show how it is now. Pay attention how well the engine DRIVES and how it holds the speed coming down on the Hour Glass and Vertical 8.
My friend had an ST G51 in this model before, but it was nose heavy. Adapting the Enya SS50 fix the CG problem and it was easy, as the Enya SS50 has very close mounting dimensions.
Martin