Any tips on how to clean and polish the cylinder fins on my bike (1975 DT125B)? I added a photo of what it currently looks like via this imgur link: imgur.com/a/01CiSnQ

You need to use a third party host such as Imgur to show photos.

As a tool, pipe cleaners would work.

For the dirt inbetween the fins, I’ve wrapped a file with 3M scrubby pads… a file would work to chase around the fins f you haven’t anything else.

My DT2 looked about like yours when I brought her home. The old baked on mud had been there for, at least, 27 years. I used an All Purpose Cleaner/Degreaser along with hot water and several small bottle brushes I bought off Amazon. After that I followed up with my Steamer and the bottle brushes again. The head and cylinder jug didn't come out perfect, but to the degree I was looking for to match the rest of the patina on the bike. For perfection I'd guess vapor blasting and a respray.