Not a tax professional, and you should get professional advice if you're in this situation; but the way I handled it was by treating it as regular income (since

Not a tax professional, and you should get professional advice if you're in this situation; but the way I handled it was by treating it as regular income (since the IRS defines "gross income" as "all income from whatever source derived"). So pay regular income tax on it, using the initial trading price to determine how much. Thenceforth it's a regular capital asset with a cost basis of whatever you used as the initial price. Essentially, it's like you found X dollars and used all of it to purchase the newly forked coin. You have to pay income tax on the found money.