Man go fer four wheels, not two.
The 89 zilla already has the better head. The 87 was the bad year. I'd pull the mag cover and check for play. If there is any, probably going to need the bottom end looked into. Zillas are notorious for hurting the mag side crank bearing.
If theres no play you should be good to go. I've owned two of them and had no problems w/either. They just need attention like any other quad. As far a feeling like a tank, They are bigger, but very stable. I'd love to have another one sitting around to play with.