Whiew! 20 years... yeah, drain the oil, and filter, pull the valve cover and make sure the cam is heavily lubed, I might even pull the rockers and GENTLY carress the lobes with fresh engine oil, polish with some light scotch brite, maybe brown... if the cam is still oily and no rust COOL, however, I bet not...
anyway,reassemble the rockers loose if you removed them. Next I would pull the GP's and spray in some PB BLaster and let it soak for a day or more, then spray in some more. pull the Injector lines and if they are dry, drip some diesel in them. again, if still wet, COOL. let the PB do it's nasty with the pan drain out, so any drip through goes out of the engine. after a day of that, put in a tblspoon of Marvel mystery oil in each cylinder and let it soak for a day, then with a crank socket... 27MM I think. GENTLY turn the motor by hand. ( the MMO can still be in there, and you DON"T wanna lock the motor or break a rod...like you could do that with a breaker bar) if it spins over GREAT, if not either let the motor sit for another day, or get a bigger breaker bar... or a cheater on it...I.E. pipe about 3 foot long... carefull, don't break the fan shroud with a slipped cheater... ask how I know. once that is done, put in the drain plug, fill with oil, put all the stuff you removed back in, set the valve lash,and crank her up. I would start with a bottle of fresh fuel hooked to the fuel lines, prime the IP, and go.
John