I don't think you would get the gains you were looking for with the major engine mods. You wouldn't find titanium engine parts that would fit your application. Because you mentioned "cheap", the cost would be prohibitive to do most of the engine mods because everything would need to be custom made.
Gearing up as high as possible doesn't guarantee efficiency. The small engine size would not be able to provide the torque needed to accelerate and you would be wide-open and bogging most of the time.
Depending on what you want to do with this, a few simple mods, moderate gear change, and a small windscreen may be a reasonable solution to achieve better efficiency.
Look over this page from Craig Vetter's website for some ideas on the mods that were done over the years to reach incredible MPG numbers - 470 MPG-Craig Vetter Fuel Economy contests, better mileage