Here's an update: With the friction wheel, the Bowser worked quite well, and then it seemed to slow down more and more. The Bowser motor had a very poor setup for the brushes.. Hard to describe. The brushes were on long copper bands that pivoted on insulated screws. Opposite the pivot points a single spring held the two brushes apart. The spring slid on a fiber(?) strip that had a lip to keep the spring from touching one brush. The spring lost tension and had trouble keeping the brushes against the commutator. Then the fiber strip failed. I took the gearbox apart and found an MTH loco motor would fit, but couldn't get the worm gear off.
I gave up and ordered a Ross conversion kit, with a gearbox, decent motor, ribbed belt, and ribbed large wooden drive gear. The gear went on the shaft with no trouble, and has a decently sized set screw. Works great. Seems to be excellent quality. Rather than using the power supply that comes with the kit and requires 120-volt access that is not available, I'm using my existing 18-volt feed for the Bowser motor, with rectifier & DPDT monetary switch, to which I'm adding a PWM control.
I'd have bought the entire turntable but it has a larger frame than the Bowser did and I can't spare the few inches. I also reworked the turntable track so it's slightly longer than the turntable, as someone did above. Not noticeable unless you look very closely, and now a few more of my large steamers can be turned.