Would need to see more of the flush valve. If it has a conventionally sized hole for that, you can probably replace the whole guts with a generic valve like the Fluidmaster. If the hole is weird, or there's no room, you may have to get parts from the manufacturer. It sounds like there's either some crud caught in the seal or the seal itself is bad. On many, that's an easy replacement, IF you can find the parts. Yours doesn't look like one of the conventionally, readily available valves, but can't be sure without seeing the whole thing.