Look in some modern one piece toilets (in the store) and see if there is a
standard kit for them. Do you tiink the leak is between the flapper and
valve seat or the seat and the tank floor?

Thanks so much for your reply, I really appreciate that. I looked at my
local Home Depot and also fluidmaster.com, couldn't find any standard
"one-piece toilet" repair kit.. I think the leak is between the flapper
and the valve seat (the brass ring), the valve seat is probably weared
out although it looks fine. I remember that I tried to wiggle the
flapper so that it would close more tightly with the valve seat (the
brass ring thingy), and at some point it worked, that the water stop
leaking into the bowl! Well, I'm hoping that I could find some type of
fluidmaster type of complete repair kit that I could repair it, and if
I'm really out of luck, I'm considering to buy either a Kohler Santa
Rosa (K-3323, one piece, $343), or a Kohler Portrait model K-3591
($427). As the bathroom was built in 1936 and we don't plan to remodel
it in the near future, I figured that if we are going to replace the
toilet, we might want to look into something more classic and the
toilet color should match with the bath tub and the bathroom sink.
Given that the trouble with one-piece toilet (harder to repair), should
I just go ahead to buy K-3591 ?? I know a lot of people like ToTo
toilet, but they don't carry toilet in "Seafoam Green" color, what are
people's recommendation about which model/brand to go for the toilet
(that's not too expensive, but has "seafoam green" color available and
has a classic look, and reliable)?? Thanks so much!