Elbow grease. Lots of it.

My bike had been sitting out for several years, leaking, burned-on crud, and more when I bought it. I took it apart and cleaned everything. I went through a lot of solvents sbd degreasers. I stripped the clear coat off all the aluminum parts, sanded (and sanded and sanded...) them, then polished. I repainted the engine. I took a brass wire wheel to all sorts of small parts and fasteners.

I don't think there's a shortcut to doing it right, so it comes down to what you hope to achieve and how much work you want to put into it.