Its hard to say with the 28k miles,it all depends on the owner that had it before you.Just like with a car if the motor wasnt taken care of you just dont now anything for sure.The top end being rebuilt could have had something to do with the plastic oil gear Kawasaki put in it going bad.That happened with mine,once the oil pressure was lost and the motor was shut off right away at the speed and rpmĀ“s my buddy was going(the previous owner)the damage had already been done,hence the factory authorized repair.Just a possibility.As far as a tune up I wouldnt fix it if it wasnt broke.I would just do the routine maintnence stuff,oil,plugs,filters and a good once over visual of all systems.Kawasaki recommends 91 octane and I know my bike runs best on it but,I have a buddy with a Classic and a Mean Streak and he runs regular 87 in both of his.He said he just started with super and worked his way down from plus to regular and never noticed a differance.So just try it one grade at a time,if you notice a differance go back up a grade.I know its been awhile since you posted and you may already know this stuff by now but just in case there ya go.