Motorcycle transmissions are not like car transmissions; you don't just buy a gearbox and whack it onto the back of the motor, because the gearbox is in unit with the engine. His "transmission ready to put in" will probably be a couple of shafts with the gears on them which you will have to assemble into the existing cases, which might have to be split (I'm not familiar with whether the ZX has vertically or horizontally-split cases) which means taking the engine out of the frame, probably, and disassembling the entire thing. So I'm wondering why he didn't just buy the second-gear set, unless this "transmission" is a second-hand one bought as a whole, in which case there might be other things wrong with it.
And how do you chip a gear without abusing the bike, anyway?
Generally, experienced riders (which this type of bike was designed for) already know how the various bikes in this class stack up against each other, so I have to say that, by asking your last question, you are demonstrating to me that you should be considering something considerably smaller at this stage of your riding career. And please don't say you've ridden dirtbikes for years - there is no comparison.
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