The seat height is the biggest difference. Then it's how far the feet can be pushed forward. I think the best way to know if it is worth the time/effort to mod the seating position is to sit over a pillow or two. The ZX was a move forward. Raise the seat further, push the feet forward, then raise to the steering and plastics and windshield to match the new seating position, and that's a Generation 1 REV. Keep pushing the feet forward to reach a Gen 2 and then I think it skips to a Gen 4.
The multiple generation REV platform is also the advent of an A frame type build to solidify the taller steering and the overall chassis. I am not expert on the different frames. I just want to make sure that I don't oversimplify the changes that sleds have taken over the years, and inadvertently provide ideas to ZX mods that are not quite right.
Just like with a car, the seat is never right for everyone. Raise the seat and the steering wheel is a little off. Someone who is 6'5" will see multiple times better improvements on a REV Gen 2 over a REV Gen 1, then another who is 5'5".
My best suggestion for ZX owners is to keep the actual seat stock and set it aside for authenticity and resale. Then mod any seat to make it fit for a better/higher seating position. An inch, two or three will be a lot more comfortable than what is there. If you only care what it looks like, then get a REV Gen 2 and up. When I see what guys on DooTalk have done to their ZX, and more commonly to their REV Gen 1 REV, I think it looks good. Who cares if it shows the majority of the gas tank. That guy maybe cares, but I don't, and many others do not either. Do what feels right, not what others think. As a good example, look at what POWERMODZ Louis Skebo has done to his sleds.
As for the engine power. I do not believe this is anything to worry about. If you look carefully across DooTalk, you will see the sleds with the gamut of power from a 600 ACE to a 500F fanner to a 600 EFI to a 900 ACE and on and on. There is no end to the snowmobile power craze. What is often found under the hood of a ZX is usually pretty stout, so a non issue. I have my favorites, but those are engines that I have grown to know better.
The one in my avatar is an original red/yellow ZX chassis with an 800 twin.