Aluminum looks beautiful when it has been freshly blasted, but the downside is that it doesn't stay that way. It corrodes quickly and will look like trash before too long. If you want a natural look, go with aluminum color paint. I have had good luck just using duplicolor aluminum from Advance Auto.
+1
i love the way soda or fine bead blasted cases look, and digger is right it will oxidize if it gets exposed to water/steam or left under a tarp. id recommend blasting the cases, and getting some high temp aluminum color and high temp clear rattle can and testing it on a piece of cardboard. see if you like the color from the can better than the engines finish. then id say take your time and clear the engine with either choice so it has a bit more protection.
here is a pic of my girlfriends 75 cb125 engine, i hand sanded and polished with rough steel wool, then shot with my airbrush. i didnt clearcoat it because i like how this finish stands.
i did take all the chrome off of my left side exhaust because there was more rust than chrome, polished with a burned out flap disc, and shot it with high temp clear. nice from afar but far from nice, still need to be replated, but check the resilience of that 3 dollar rattle can clear.
good luck bud