mfzx6r wrote:Some people have put early model zx9r motors into zx7r's
Aye. The ZX-9R-B motor is a drop-in swap into a ZXR750-J or -L (give or take a ding in the underside of the left frame beam to accomodate the oil feed banjo on the front left corner of the ZX9 head), but it requires the top engine mounts to be cut off and relocated on the ZX-7R.
Going further back, the GPz1000RX motor will go into a GPz900R.
Given enough time with a blowtorch and a mill, anything can be squeezed into anything, really. One issue of Performance Bikes featured a ZXR750R with a ZX-9R-C motor in it, and one of the members of the FZR600 mailing list had a pair of FZR400 racebikes with FZR1000 motors shoehorned into them somehow.