I'd be tempted to track down one long enough that when you made it small enough for your new set-up you'd have enough left over the lengthen the one you have. Some places that sell chains online list the lengths. You'll need a new chain in a few thousand miles anyway. Better to replace the chain as soon as it is worn rather than have the worn chain wear out the more expensive cassette and chainrings. If you don't have one, you'll need a chainbreaker, which is a good tool to have anyway, and a masterlink. I tend to carry a chainbreaker (as part of a multi-tool) and a spare masterlink anyway.