That is a Kohler "canister style" flush valve.
First, carefully remove the "rim-clean/bowl-fill" water supply hose (the little black hose that runs from your fill valve to the flush valve. Also, remove the chain that attaches the trip lever to the flush valve.
To get at the gasket in question (which is very likely, but not 100% certainly, your problem) ...
I agree with Sfors on trying to seal up the leak areas, as this will make plunging a future clog in the primary drain possible.
Although, the rust and leaks being that high up tells me your overflow's outlet into the primary drain is blocked and the overflow is staying mostly full, instead of freely draining straight through.
The overflow's outlet is a ...
Sealing that joint is a low-percentage game. The patio probably moves with seasonal changes in ground temperature, and most products will let go after a few years, leaving you in the same position.
I'd do this:
Remove some decking to make access to the patio.
Using a rented diamond saw (chain saw style) or jackhammer, cut 2-3' off the foundation side of ...