On the run-up to Dirt Quake 1, I recommended the Mitas Speedway rear as perfect cheap option dirt track tyre. One of the only drawbacks is that it's not road legal. As I'm planing to get an MOT for my chop but want to maintain the bobbly* look, this Taiwanese trail pattern Cheng Shin seems like a good choice. Unlike a motocross tyre, it has the same razor longitudinal cuts as the speedway rubber which give some extra flexibility in the knobs.
At 3.00 x 19" it's slightly narrower which makes for better handling on the street too. It cost me £50 at Kempton (which in retrospect sounds a little dear). I haven't tried it in action yet but the compound feels nice and grippy - unlike the perished and hard speedway cast-off I ran last year.
Your back tyre isn't so crucial if it fishtails in the quagmire but if you want to keep your pants soil free, it's most helpful you have a well planted front-end. BP
This Evo' built by Benny Boneshaker is a perfect example of the genre.