A basic Marbles tang sight might suit your needs. Perhaps the Marbles Bullseye sight? This one fits into the rear sight notch on the barrel. Or Williams? Then there's Skinner, XS, Ranger Point, and Wild West Guns, if you have deeper pockets. Ranger Point does make a combination picatinny rail/ghost ring sight. But, I don't believe one that fits the '94. Model 336. 1895, yes. Also pricey. Hope this helps.
Edit: Just took a quick look at the Williams site. Williams does build a combo picatinny/rear peep sight. Called the : "Ace in the Hole" May be worth a look.
I also agree with KaneZor....Skinner is the way to go. Very rugged sights.