I would say that if you want to try the engine again,take it out of the heli and mount it on a test bench and put a prop on it, a 9x6 prop should be fine.Try get the engine running on the bench,it will be easier to tune as well.Set all the needles to default as per instruction manual and start from there.Use at least 5-10%nitro,it will make tuning the engine easier.Make sure the tank is getting pressurized from the exhaust and change things like fuel tubing and glow plug if you are not successful,then resort to cleaning the carb out in case there is dirt in it.The engine is not that bad, Super Tigre's are good engines IMO although I am not familiar with the heli ones. O.S is undoubtedly the best, I use an O.S 32SX in my Shuttle ZX and haven't had one flame-out.When I bought my Shuttle the engine had been standing for 5years, I bought it even though it felt like it was ceased but I knew it was the fuel that got all sticky and prevented the crankshaft from turning.I flushed it with fresh fuel and it slowly started to move again,then ran it on the bench shortly before sticking it back in the heli.Worked perfectly ever since then.Just remember to replace things like fuel tubing etc.

Regarding the setup of your heli,the pitch may be too high and therefore stall the engine.Use a decent pitch gauge to set your pitch.The paper pitch gauge that comes with the Shuttle is a bit useless, it works but is not very accurate.You should have 0 deg at bottom, +5.5 degrees at the centre and about +9deg at the top.This is just for hovering so don't go too high because it will take long to come down with no negative pitch.The reason for 0 deg at the bottom is so that you don't slam the heli down when learning to hover.Don't forget your training gear!

I wouldn't bother getting a new engine yet until you can hover properly because you won't fly high anyway unless the engine is really bad and you can't get it to run at all.Hovering is not so good for engines because they don't get sufficient cooling and are therefore more likely to break.Get a new engine the moment you begin forward flight.

Good Luck!