Kind of a neat trick if you've got the top end of your motor off is to hex shape your cylinder and cylinder head.
By taking some material off of them, it gets into a thicker part of the fins. In factory state the fins come to a point, more or less.
That makes it hard to paint and wipe them. Not hard to do necessarily, but hard to make them look good. When you grind into them a little , there becomes a larger surface to be polished.

While you're at it, I think it looks tricker to hex them as well.. Something a little different than the norm.
I've found it's easier to use a pedestal belt sander if you can. I've tried using a body grinder to rough it in and that works ok, but  with the belt sander you're manipulating the part itself and you seem to be able to control the process better with better results. Thats what worked best for me any way. Once you have them the desired shape and the fins are somewhat thicker, you can begin to detail the fins with sandpaper or emery cloth and then polish them.
Once you're at that point you can either mask the fins or paint the parts and wipe the fins off.

In the examples I've shown below, the parts have been chromed, but they give you an idea of what it will look like.