There are too many variables here as to model type and condition for me to give you a good answer. The first step is to get on a vintage Yamaha motorcycle enthusiast's site (yamahaclub.com looks promising) and post a picture of the bike with the VIN and model name.
Even if you could get this bike started (which I doubt) it's NOT safe to ride if it hasn't run for 30 years, so don't do it! The tires, brakes and electrics are all questionable. Actually, the tires are unquestionably dangerous if they are original.
I think you will find that the restoration costs far exceed the value and hassle. I'd donate it and take the write-off.