Cool project. Nice work on the bars/grips & congrats on the cotter pin removal. There are few C&V delights comparable to unsticking a stuck part. Btw, don't throw that cotter pin away. You may be able to salvage it. If not, it may prove useful in matching the cotter face angle should you need to file a new cotter pin.
Now that you have the chaincase removed, you may consider dropping it in a weak concentration of Oxalic Acid (avail at paint stores such as Sherman Williams, or sold as wood bleach in many hardware stores), followed by Meguiar's Scratch X as suggested above to bring the pain back to life.
If you don't want to shell out $50+ for the bikesmith cotter press, I found success with
a vise and a socket.
You'll be replacing the chain, right?