I have seen two different primary decals that come up on eBay all the time. One is a complete "rectangular oval"; the other has a cutout for the footpeg support. My 1980 takes the latter. I would guess that the 1971 takes the former. I believe it just says "Sportster" on it, not 900 or 1000. Do you have a FM? Or some other publication with a pic of a '71 to see what it should look like?
The decal is the same as the 1000, but says 900. I found one for $55.00, but that seema a little pricey.
That would be the going price for a rare item. In excellent condition it might be worth it if you are restoring. Otherwise one that says Sportster rather than 900 or 1000. Just be sure to get one with or without the pedal cutout as your bike needs.