engine swap

Can a engine from a 76 or 77 kz900 bolt into a frame of a 1981 csr 1000?

It can be made to fit but it's norpt a simple unbolt the old one a d bolt in the new one. The J-model series engine (like comes in the 81+ bikes) use a different mounting setup.

Later, Doug

The J motor are also upgraded, the head breathes better, trans is beefier, better motor IMHO. Get a J motor if you can

I only ask cause I have the 81 and a friend would like the bike to put his older engine in. How much fab work does it take to adapt the engine into that frame??

You need to fab new front, and rear motor mounts along with making a complete NEW bottom mount (81 and up models have no bottom motor mount) this doesn't take into accound lining the motor up correctly for chain pull and all else, would likely need to get measurements from a pre 81 model and see if you can get the output shaft to swingarm pivot in the same alignment. One way works (J motor in kickstart frame? Or kickstart motor in J frame? I'm not sure, the other way is a lot more involved due to size differences)

Like I said, may be easier to find the correct replacement for the bike.