If you happen to see a good deal on a used drz250 engine, let me know. I've searched for one, and they are not very common.
I have found a LT250S engine, that I'm working on. The plan for it, is to bore it out to a 260-270, Stage 2 cam and have the head ported.
That won't get it to keep up with the z400, but it will at least be fun to ride, and shouldn't break the bank too badly.