try Alex Tillman at Train-Li. He is generally able to source LGB parts. The analogue board is a two-piece unit and will not be cheap. Easiest solution (and cheapest) is either a sound board from a 2-4-0 LGB 'shorty' sound tender, around $90.00 or a LGB #65001 American steam digital unit ($180.00)plus the #65011 sound storage unit ($75.00). Alternatively, purchase a replacement used Mogul sound tender and swap over the parts that you need and then sell as a non-sound version to recoup some cost, if you wish to keep the original loco stock standard.
The former two methods will not facilitate the axle chuff synching available with the Mogul as the units are voltage activated. The #65001 digital sound unit will require programming for near enough synching. Another alternative is to fit Phoenix sound, with a PB-9 kit from Ridge Road for around $209.00.