$35 is a fine price. I had a lot of fun kitbashing my Mantua Pacific. I used "Exclusive Retrofit Can Motor" kit part number 812 which did wonderful things for low speed running. Yardbird, follow the "remotor" button and they have two remoter kits, model D8120 and a helix humper. The D8120 looks like the Mantua P/N 812 which I used. The Helix Humper has a good rep. Either one, you can't go wrong. I replaced the unexciting Mantua plastic pilot casting with brass detail parts, a pilot, twin air pumps, a twin air pump shield, and a coupler lift bar. I cut a piece of one inch brass bar stock for the frame of the thing, and then soft soldered the pilot and the air pumps and shields to the bar. A #4 Kadee coupler fits thru the coupler pocket on the cast pilot. The all brass front end of the locomotive looks much better than the stock plastic casting. There is some extra room inside the boiler which I filled up with lead to improve the pulling power. I also sandwiched a bit of lead sheet onto the top of the pilot truck to put more weight onto the pilot wheels. I also changed the drivers from the stock boxpoc style to the more traditional spoked sort. That one change made the locomotive into a decent model of a B&M P4.
David Starr