ORIGINAL: ChuckW
As others have said, you can't go wrong with OS. I have two .46-AX's. Both start easy and run excellent. I also had a .46-LA on a Tower trainer ARF a while ago. It was a good combination.
I agree with ChuckW, I also have two O.S. 46AX engines. I had one in a GP Trainer 40 which is the same as the Tower Trainer 40. I had no problem with the plane being nose heavy. You just have to configure the plane correctly. Obviously bigedmustafa has a problem with the weight of the 46 AX as he has repaeted it several times in this post. As for putting a smaller engine in keep in mind where you will be fying. If flying off grass you will need more power than flying off pavement or any other like surface. Nice thing about the 46 AX when you outgrow your trainer by the end of the summer you can put in something nicer. A cheaper engine like the 46 LA will not have as much power and will not last as long.