It is possible to rebuild the original tuner if the chassis hasn't been cut to make room for the new tuner.
Our database has a total of 68 surviving TRK-12/120s, by far the highest number of any prewar sets. About 1800 were made.
http://www.earlytelevision.org/prewar_database.html
The modification for FM sound refers to FM sound for the television. Prior to 1941, TV sound was AM, and some prewar sets were modified to add FM detectors. Restoring the AM detector is usually easy.
As for value, a nice, complete set usually sells for about $8,000. The modifications affect the appearance of the set, which hurts value more than if they were only internal changes. The cabinet of this set looks pretty good. I would guess that it would sell for between $4,000 and $6,000, depending on the condition of the chassis. However, prices of old TV sets are all over the place, depending on who is looking for a particular set at any given time.