IF i recall correctly, the date stamp is in FRONT of the bracket on the top surface of the tank. IF i recall correctly.......
If it were me, i would use all the parts i can from 97, assuming they can be salvaged, repaired, reconditioned, etc, what have you. This way, when you are done, you KNOW all the parts which came with 97, remained with 97. You'll already be replacing enough parts on 97 that were damaged beyond repair or missing.
You can test the integrity of the oil tank by taking a 14mm combination wrench or a hard plastic head hammer and tapping tank surfaces to see if there are any more soft spots. If there are too many rotten areas, then you can decide to salvage tank, or not.
I ALWAYS try to use all the parts which came with the bike, unless they cannot be salvaged back to serviceable condition.
Just my 2 cents worth.
Looks to me the two halves of 97's tank are hand welded together and the two halves of 1553's tank are "stamp press welded" together.........
I don't see the exact area where you suggest there is a weld repair.......? are you referring to the seam or another part of the tank ?