If you go with the S7, I'd recommend whatever pipe it is that RB says to go with.

Oh, and I should have mentioned the Hyper 21 as well. The reason that I didn't is that I usually have trouble with the carbs on those. What I do is to put an O.S. RG carb on it and all of the tuning woes go away. These engines run really well. The only other problem with these engines for me is that they come with pull starters and I prefer to use a starter box. On the other hand, you can get the ho bao (I think) branded engines and they are available without the pull starter. They are pretty darned fast, but for some reason I just don't see too many of them, so I just don't think about them much.