Exciting Toilet question, can you advise?

[Wash. state Desk Jet]

5. Given the age of the toilet

it may be even a new flapper cannot seal around the flush valve seat because the flush valve seat is worn and uneven. When for example a toilet is rebuilt all parts in the tank and tank to bowl are replaced.

There is however a repair kit you can buy at HD or lowe's that slips on to the top of the flush valve seat thus allowing the flapper to seat itself correctly. That or rebuild the tank by replacing all inner tank parts and tank to bowl parts. Since you already replaced the fill valve ,that part isn't necessary. The fill tube and the the seat the flapper seals off are connected or one piece.

If you have never rebuilt that tank and tank to bowl parts meaning replaced all parts,, it just may be about time you do that.

Or get the repair kit for the flush valve seat, about $ 8.00

Good luck.