I'd have to second the bigger and better looking engine comment, if all of life's upgrades could be that easy...
Best money spent would be a jackshaft, with that I never had any hill that I could not climb with a hundred pounds of groceries in a cart behind me.
Second best is the right oil, the cheap stuff let's your rings eat away at the chrome a lot faster, and everything else. Organic oils tend to get all sticky and block up holes and ports with gunk and carbon. I learned that the hard way. I don't know what is the best synthetic, everyone swears by something different but I use redline now.