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I don't know if this is still the case, but a decade ago titanium valves were a Very Bad Idea on anything but race motors built for very limited duration (like 3000 miles or less). Titanium loves to gall on just about everything, and the valve keepers were real trouble sites.
You would want to deal with airflow management inside the engine as well. Those pistons moving up and down make a good bit of air move around inside the case, and the crank and rods churn it all around as well. So doing stuff like "knife-edging" the crank, and making sure the air has good ways to flow around things, will help cut down on power lost to moving the air around. Drawing a vacuum on the case helps that as well, but it costs you power to pull the vacuum.
The oiling system in the stock motor is likely intended to provide more capacity than a low-RPM eco-driven motor would need. So downsizing on the pump can help reduce the power used to drive it, without risking the engine. Especially if you make sure that the oil passages are port-matched to each other. (We have seen significant mis-matches in the aircooled VW world, and matching the passages in the pump and the case to each other has helped.)
Coatings can help as well. I am told that an oil-shedding coating on the parts of the bottom end that don't need oil on them helps reduce the power lost to flinging oil around inside the sump. Friction-reducing coatings can be used in some areas as well.
Most of the changes I am talking about are going to give very very small results, likely less than 1% improvement each. They are likely past the point of diminishing financial returns until fuel costs get into the truly absurd levels. But they are interesting as an exercise.
-soD
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