Anyone know if the gearbox on an FZR400 is the same as an FZR600 gearbox, also selector forks etc?

My bike is sometimes jumping out of 1st gear, it jumps out at quite low revs.
A couple of weeks ago I went to pull away from lights but was in 2nd, so I knocked it down into 1st, but it crunched quite loudly. Since then it has started jumping out. I'm thinking it could be a selector fork.
I have read elsewhere that it's possible to drop the sump and change the selector forks and the selector drum without a strip down, can anyone comfirm this?

The crank cases are common 1WG = 3HE and 3TJ = 600R / T'cat but beyond that I couldn't say.

I've got a spare 1wg and also a spare 3he, just wondered before i start stripping one or the other

I woudn't like to say if the 3he selector fork would be a swap.