I think it is very easy to replace or repair the smoke units in berkshire Jr. locomotives. You may not need to replace the whole unit though. You may want to try replacing the batting and resistor. I had problem on one of the new berkshire jrs with whistle steam (the Polar Express one). It was a simple fix I cut the wires on the smoke unit (the two coming out) replaced the old resistor with a 27 OHM and put in new batting. I replaced the batting with pink fiberglass insulation and it worked well. Male sure to properly insulate the wires so a short does not occur. I also used mega steam smoke fluid which seems to work very well in Lionel smoke units. I have heard a lot ofthese units do not smoke well because of a ceramic 30 OHM esistor I have found this to be true on many locomotives. Here are videos of the two I put 27 OHM resistors and pink insulation in. Igf you do need to replace the whole unit that is also easy you just cut the wires and remove the screws holding the unit to the frame and put in the new one and rewire it. they are on Lionel website for 12.50. Hope this helps
I agree, unless something is broken in the piston or chamber, all you need to consider is the smoke element as a rule. If you unsolder the wire from the frame, you can lift the whole top out and replace the smoke resistor.