20 year old gas is probably still good, the newer stuff dies after a year or so. Drain it if you are bored but when I got the '76 Chev van started that had been parked since the late '90s the fuel was still fine. My major concern in your case would be brakes. The rubber in the wheel cylinders/master cylinder may or may not fail after the first few times you put pressure on it. Don't bother flushing the brake fluid, just keep an eye on the level and look at the backing plates of the wheels and the base of the master cylinder the first few times you drive it. You might get lucky and everything will just work fine, or something may leak, you won't know until you try it. And be prepared for the float in the carb to get stuck, causing the carb to spew gas everywhere, keep something like a screwdriver handy to wack the bowl on the carb to free it up so you don't end up starting a fire. And a $2 inline fuel filter spliced into the rubber line running from the frame to the fuel pump could save you a ton of grief in case there is rust or other junk floating around in the fuel tank. Good luck!
