Then just pick any date, or go with something like 999 or AAA - take steps to make sure no one is fooled.
I have stuff (original stuff) here for a few cars that currently have fake tags. I do not let the owners know I have the stuff if they have tried to fool people with fake tags. I can wait until the car has a new, more honest owner.
Since your original tag is missing, you may want one that looks like this;
+ Reward for original fender +
tag or broadcast sheet
for BS23V0B137160 call
(insert phone number here)
That way, you have a tag to place in the missing spot, & when folks see it at shows, it may just lead to something "real" for your car.