What head gasket did you use?
If the new head gasket was white in colour then it's probably the same culprit as the cheap head gaskets simply do not work. Use genuine, Athena or Vertex head gaskets only.
Did you install the head gasket with UP facing you? The head gaskets can be accidentally fitted upside down, which again will cause similar problems.
When you say New coil you mean new genuine coil. Again the knock off chinese Racing coils DO NOT work. So only use genuine ignition coils.
I don't know what a HPI ignition is, but if you can source an OEM ignition system you can rule out faulty electronics.
Common problems on these engines will be things like, waterpump seal leaking into crank case and then crank seals leaking into engine combustion chamber. Both are pretty obvious but will cause running problems. And when I say common, they don't come up that often as the engines are pretty darn bullet proof.
