The manifold I suggested (Novarossi 41032) is just the manifold, not the whole engine. The manifold will fit almost any engine. As I said (and if you looked at Neal's dyno graph) the manifold can have a significant impact on the power of the engine. You wanted more top end, and that manifold will likely do that for you on the same pipe even. Not a guarantee, but the conical manifolds make a nice power boost usually. But you'll have to see if your new pipe/header works better than the one you have now.

On a single cylinder engine, manifold and header are used interchangeably. Generally headers are tubular in shape and manifolds tend to be "blocky" cast pieces. Headers (tubular) will flow better and smoother than a manifold and the conical headers will flow better yet and allow the exhaust gasses to transition to the pipe's first chamber much smoother (in a nutshell will boost power).

I have found that that if the manifold is too long, the engine won't rev as hard, will have less top end, and the engine can run hot. How the engine responds to the pipe/header is dependent on the timing of the engine, size of the carb, length/shape of header, and pipe chamber shape/volume. Lots of variables to consider - throwing parts at an engine can lead to disappointment when your particular engine doesn't respond as well as other engines. Do some googling on pipe/header combos on RCTech. I think there's much that can be learned.