VANOS (Variable Valve Timing) to convert would be an intense & expensive project with no real benefit. It's a slightly different head, camshafts, timing gears, chains, some oil line plumbing, a solenoid and a new DME. The only thing a VANOS system does is it adjusts intake camshaft timing based on engine load, engine speed & temp. You'ld only gain a smoother "mid range" in simple terms.
If you want Vanos get a motor already equipped with it. As "Dough" mention in his case to repair would be $630 but he has all the original parts - you would be looking close to 2k to convert. FI would be a better choice for power/torque etc.