Hi Jeff, you have lots of options here! It kind of depends on what kind of riding you are doing. If it's "trials-type" riding, you would want to stick with a trials tire.
I've been told that the Michelin X-11 radial trials tire can take a tube - but it is expensive. Dunlop or IRC would be good trials tire choices as well. If you're riding trails & single-track, the Dunlop may be better - it is said to have a stiffer sidewall (see link below).
https://www.dirtrider.com/tests/parts-accessories/141_0705_trials_tire_comparison/
FWIW, the Michelin on my trials bike is awesome!
If you are riding single-track, here are a couple of other options - but I personally will keep a trials tire on my trials bike; it just "works". The only reason I can think of to switch to another type of tire is if you are in lots of mud. The trials tires don't have much room between the lugs for the mud to "clean out", and once the voids fill up you lose traction.
One option would be the Kenda Equilibrium. It's a cross between a trials tire and a knobby. Great grip, but will fill up with mud like the trials tire will (I have one on my '05 enduro bike).
Next step toward a knobby would be the Shinko 505. If I wasn't going to put on a trials tire, this is the one I would recommend. Gummy compound, large and closely spaced knobs, doesn't attract mud like the Equilibrium does, excellent grip on a variety of surfaces.
For a full knobby, the Kenda Ibex would be a good choice. Gummy compound, great grip, and will "clean out" well. But it might not fit!
Do a search and you can read forever!
BTW the only trials bike that I have heard of with knobbies is the Beta Evo Sport. So it has been done, but is unusual.
To end, I will reiterate that I don't think you'll be happy with a knobby tire on your Contact. Get a good tube-type trials tire and go
Jeff
P.S. This guy's comments ring true to me:
http://www.trialsnuts.com/faqs.htm
Q: I want to put a knobby tire on my bike because it will get lots better traction.
A: ...the trials tire grips better. Keep in mind you'll need a small knobby because the swingarm is narrow and a big one just won't fit. Also, you'll need to run a tube. And you'll need to drill a hole in your rim and install a rim lock. ...I'll tell you about the only place you'll see an advantage with a knobby is in loose sand. Guys with enduro/desert bikes are aware of this - that's why a lot of them are looking for used competition trials tires and installing them on their KDXs, KTMs, etc. Trials tires are better on hardpan, rocks, just about everywhere but loose sand. And if you're primary riding terrain is deep loose sand, you've purchased the wrong kind of bike! Again, I'm talking about competition trials tires, not el-cheapo tires with a "trials" tread pattern. Those things are not in the same league. I've said my peace...