The standard ECU can be remapped, whoever told you that it can't is an idiot. Wayne does them for example, in t'north
The cams should match the inlet manifold you have. If you have a J4 inlet manifold, use the J4 cams. If you have the converted C2 manifold, use the JP4 cams. Aftermarket cams are available for that engine but it's not worth it unless you're racing.
Cams NEVER give you extra power, what they do is rob bottom end torque and add a tiny amount of HP to the top end.
The easiest route for a bit of extra power is a set of throttle bodies and a good quality aftermarket exhaust.