kylaeyadon, looks like your doing a lot of interesting and fun mods.
I have been modding my auto mazda 626 so you might be able to find some useful "easy" and cheap mods if you look at what i have done to my car (bugler). I have listed all my mods under the car garage so you should be able to see them.
The mechanical changes you are proposing sound like a very big job so i wish you good luck there and i look forward to seeing how you go.
I do wonder if the lighter pistons will do anything for you. From theory it seems they will just reduce engine inertia which i think would cause an incredibly small impact on FE if any.
Also increasing displacement will increase your fuel usage not decrease. A larger displacement will cause more air to be sucked in and out of the engine causing higher losses in efficiency. If anything you will want shorter stroke pistons which will mean you will have to change the height of the piston rods, the crankshaft and the position of the crankshaft or grind down the engine block. When i looked into this myself i decided it was too difficult and expensive.
so as for my not too informed opinion i would suggest these mods would not be beneficial but i am happy to be proven wrong
