Welcome to the insanity that is owning a sandcast! Ha!
All kidding aside, congrats on getting one of these special bikes.
Your bike shipped with a wrinkle tank in 1969, so indeed it should have one to be "correct". Wrinkle tanks go for anywhere between $400 (in bad condition) to $2000 or more under the right circumstances. I suspect that a restored bike that doesn't have a wrinkle tank is actually dinged a bit more than the value of the wrinkle tank simply because of the perception that it is not correct. Just a guess.
A few suggestions:
1. Camp out on eBay and wait for a wrinkle tank to come up. Have it painted and you will be golden.
2. Buy a reproduction wrinkle tank from Yamiya. Of lower value...but much cheaper than an original. At least that way you can point to the wrinkles when someone asks
3. Don't worry about it unless you really need to sell the bike for maximum value. You can't tell your bike has a later tank until you go to fill the tank with fuel...which means you are riding the bike...which is the point after all!