Looks like a 1979-1981 tank.

 

It's from around 1988. I have one with the same design (different colors) on my bike, and it came off a 1988 model.

 

The design on that tank is very similiar, but different enough that it could be from another year/era as the OP's. Or, maybe not.

 

It doesn't have a bayonette filler neck so it can't be all that old...but the right side petcock indicates some age. But to answer your petcock & cap question, you can use a 22mm petcock and cap from a 91 to 93 if your tank has the male fitting on the tank. That's the same tank I have on the bike in my sig pic.

 

After doing a little online research, I'd say there's a very good chance that this particular design dates to the 1990 models. My original guess was around 1988, so I wasn't too far off...