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2 more cents,
I expect the low hanging fruit probably includes high compression, and modern electronic fuel injection.
The common cam swap on this forum which is proven is the Geo Metro XFi cam into the other Metro engines. The XFi cam (to my knowledge) reduces max power. It doesn't breath well enough at higher RPM and reduces max horsepower, but it improves economy at low RPM. I'm unsure if it actually improves torque. You might look into exactly how the cam profile differs to get an idea of what works in that engine.
Honda is most famous for its VTEC, which is just having two different cam "settings". As I understand it, at low RPM, there's a low lift cam profile which is used which promotes swirl (one valve opens only partially) and gives a smooth idle and better combustion. At higher RPM it shifts to a high lift cam which reduces economy, but allows the small engine to get a lot more air into the cylinders and improve horsepower. This let Honda retain small displacement (helps economy) with a smooth idle and good low RPM operation, while having as much peak power as other manufacturers' slightly larger engines.
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