Kawasaki FD661 Replacement

[apeters]

We have a 1998 SKAG with a Kawasaki FD661 engine.

This engine is out of production for now.

1) I have read that other 600 series horizontal water-cooled engines will fit? Is that true?

2) If so, will other 600 series horizontal air-cooled engines do the same?

3) What are the changes that need to be made to use other 600 series engines?

Thanks

[bvillefarm]

What is the spec number?