Hi am new to this forum, i have a 1989 cbr 400rr tri arm am looking into putting a cbr 600 engine in it out of a jelly mold model cbr 600, just wanted to no if anyone has done or tried this before?? I dont really want to have to do anythink to my frame to make it fit incase i want original engine back in. Also if it does go in what would i need? Ie would i need the full loom inc cdi etc etc, exhaust? Anyone that could shed some light i would be very thankfull

You need to cut the frame to get it in, forget it.

Yes you need to cut the frame.
Google cbr400/600 conversions as you will find quite a lot on the subject

If you really want to put the 400 engine back in then all you would have to do to the frame is machine some spacers up and get longer bolts for the head mounts. There are quite a few parts you would need to have off both the 600 and the 400 to do both conversions. Good luck anyhow.