I was thinking about this, looks like the 140 came with the aluminum 42" deck i believe 68-71. thread with pics-
Video of one working on a 140 with elec/hydro lift. Also called a magnesium deck-
Also manuals here might find some more info
To mount a 120 deck there should be a rear bracket under the frame below the battery. Also a front bracket with 2 spring-loaded hooks under the engine. Deck has a scissor frame that hooks into the 2 tractor brackets.
The older steel deck may use the scissor mount. 65-67 i think had a 36" square steel deck. Might be the one on the tractor up north. I think the newer steel decks were bigger than the 42alu. As others have said i wouldn't pay more than 50-75 for a steel deck and it should have the mounts to fit your tractor. Personally i would look for an aluminum deck, powerful and don't rust out. lots of them made they pop up pretty regular.
If you want i can post some pics of my 120 so you can see if yours has the same brackets. Then you could see if a 120 deck fits before taking a road trip. Could also check out something on the main for you, i'm not far from Rockland. We get over to vinalhaven sometimes to see an old friend, could carry my deck & you could see if it works on your 140, see what to look for. Just have to wait until spring and get the logs out the back of the truck, iced in pretty good now. good luck with your search-