Do you want a close ratio gear box or a wide ratio, or could you get by with either?
FWIW, the cross country racer I bought my 2002 KTM 250EXC (2 stroke, enduro bike) occasionally motocrossed it. It has an outstanding motor for a 250 enduro that can keep up with the true MX bikes.
I used to own a full enduro conversion '99 Honda CRE250. It was an MX bike that was an absolute joy to trail ride, and the guy I bought it from won a few enduros on it.
While you have put a 250cc limit, the KTM300 might also be a good candidate.
If you want a lighter, smaller bike that is a rewards agressive riding, the KTM200 might also work for you. They are a blast to ride.