American Standard ....tank guts

I have an old american standard one piece toliet. I need to replace the tank guts but do not know where to look. The tank has holes in the back of it and the height of water below the hole is 7" max. The house was built in 1978 and I believe these to be the original toliets. Any ideas?

Standard

It may need a #4 Am. Std. replacement flush valve asse.

I believe a Fluidmaster ballcock will adjust to the level of water you need.