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Looks like photos I've seen of carp pox, but I have no personal familiarity with it to say for sure. If it is, there is no known cure. Most will say that it will take care of itself with time and warmer temperatures, but will come back when the water cools. In my pond, the fish would be euthanized to prevent it spreading. Others would disagree.
As for the ulcer, check koivet.com for courses of treatment. The symptoms page will lead you to the basic steps.
You should check all your water parameters, filters, etc. and take whatever steps are needed to have top quality water and biofiltration. The conditions which caused this fish to develop an ulcer are affecting the others. You can expect the outbreak to spread unless steps are taken to maximize water quality.
This is the time of year in the northern hemisphere when these sorts of problems are seen. They usually reflect the koi having endured winter in too cold water with compromised filtration. In your case, it may be a secondary consequence of the carp pox (?) infection, but I do not know enough about it to give useful guesses.
Good luck!
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