After studying pictures, and part numbers, it looks like it'll fit in the 250 frame. @
Lychee mentions the exact same reference points on the newgen. I don't think it'll work on a pregen motor since the mounting points are different:
http://www.ninjette.org/forums/showthread.php?t=117632....unless you swap over the front engine mounts from that engine generation.
The intake side of the head of the motor may possibly be the same diameter as the 250 motor. If you look at the 300 TB boots, it necks down from large to small. You
COULD possibly reuse your 250 carbs and rejet, but why bother when you can have full EFI!
It makes me giddy thinking I can do a complete 300 motor swap in my 250
Top 3 pictures are the newgen front motor mounts.
Then 2 pregen motor mounts and then 4 300 motor mounts. I'll have to see if the pre pregen motor mounts are similar.