Fortunately the Visiton site gives speaker specs. Looking over all the 2"'speakers this one stands out. The FRS-5X. Sensitivity is 86db/watt. Resonant frequency is 190 hz. Rare for a 2" speaker to be below 200 hz and most are 250 or higher. Below that number it begins to roll off. It actually looks very similar to Lionel's 50mm Fatboy though I have no idea if they are the same.
https://www.parts-express.com/...-5x-8-spec-sheet.pdf
Look at the spec sheets and compare frequency response and sensitivity. Lionel/ERR sound boards only put out about 1 watt so any speaker rated at 1 watt or greater will be OK. This one is rated for 5 watts so no worries here.
I may have try a some of these.
Pete