While the Bachmann Ten-Wheeler does have Walschaert valve gear, it is still a slide, or D-valve locomotive. It would be a relatively simple task to remove the most obvious parts: from the eccentric crank on the second driver all the way to the union link on the crosshead. You'd need to fashion a support for the rear end of the valve stem, but could probably utilize part of the valve gear frame for that.
There were even slide valve locos with Baker valve gear, and, conversely, piston valve locos controlled by Stephenson gear. Although it is commonly believed that piston valves were introduced with the advent of superheating, they were first used in 1832, while superheating wasn't in general use until about 1910. N&W operated almost 650 piston valve-equipped locos before obtaining its first superheater.
I think that the Bachmann loco could stand in for your prototype just as ably as the Roundhouse "Old Timer", and may be even more attractive if you plan on using DCC.
Wayne