KZ400B1 Gear change shaft seal

Is it possible to change the gear change shaft seal without removing the engine, can I just remove the sprocket cover, hook the old seal out and press a new one in?

I would get the new seal, making sure it is actually the correct one by Kawasaki part number etc.
Make sure the outside is just a smooth cylindrical shape. That is, it shouldn't have a ridge around it like a belt or a lip.

Measure the O.D. Then look at the cover. If the opening in the cover appears to be about the same diameter, I would assume the seal can be pulled out. The seal will probably have an internal metal cylinder to hold its shape so you will need to make a puller of some sort to pull it straight out so as to not gouge the cover. If the opening of the cover is significantly smaller, then it probably has a lip and the seal won't pull out. It will have to push in, with the cover off.

There could also be a lip on the inside of the cover that stops the seal from falling in, which won't stop you from pulling the seal out, but be aware it may be there so you don't damage it.

Sorry I don't have the definitive answer... just saying what I would do if I can't get an answer.

Also checking photos online of used covers might show if there is a lip. Even a parts drawing might show it, if it's got good detail.

When it's all done, you will want to check the sprocket cover opening for the shifter shaft. If it's worn out, you will want to insert a piece of brass shim stock into the worn part to make the shifter less sloppy. This will save the new seal from leaking prematurely. I did this 25 years ago on a 550 and it stopped the seal from leaking. I used a piece of brass shim (maybe it was aluminum) about 1/4" wide and just bent the end to stop it from moving in too far and the shift lever stops it from falling out. I greased all of it, of course.

You were right Loudhvx. I ordered a seal, it was not a Kawasaki one but it looked like a better design as the Kawasaki one had a steel outer so could not seal to the alloy engine case.
I hooked the old seal out and made a tool from a piece of steel pipe to push the new one in place.