Engine for '98 220 frame?

I might have found a good deal on a '98 KDX220 roller, and was wondering what engines will directly bolt-in to it (as in what year KDX's, as well as if there are any other odd-ball engines that will work by some fluke of engineering)?

Also curious how the '98 KDX in compared to the more modern bikes for trails and the occasional MX tracks. I've been looking mainly at Honda CR250's and KTM MXC 300's.

Thanks in advance.

-Cheers