Probably not worth it, even IF it is possible.  Sell the set engine and buy what you want, save the work.  Lionel does not make the LC+ board available as far as I know.

You can significantly enhance the performance of the standard LC by simply soldering two 3A diodes inline in the motor positive lead.  Cut shorter, then be sure and insulate the leads using shrink tubing.  Simple 10 minute project.  AFTER the warranty expires!!!

The diodes make the engine start at about 20% throttle, and creep, way better than stock (of course the top end is also reduced.)  I can use my PA Flyer as a switch engine, because of this mod.

The diodes go in soldered Male end to Male end or F to F, so that they will work, independently,  in both engine directions.

Digi-Key Part Number	1N5820GOS-ND
Quantity Available	1,766  Can ship immediately
Manufacturer	ON Semiconductor
Manufacturer Part Number	1N5820G
Description	DIODE SCHOTTKY 20V 3A DO201AD
Detailed Description	Diode Schottky 20V 3A Through Hole DO-201AD

What is the reasoning behind the schottky diode?   Couldn't you do the same thing with one silicon diode and get the same voltage drop or close to it instead of two schottky's ?

Yes-  I actually used two random diodes from my parts box, but the DigiKey ones seem to be readily available, if "overkill".  You would need TWO though, soldered as above, or the engine would be prevented from backing up, right??

I misunderstood you.  Thought you were putting 2 sets of opposing paired diodes in series and I was just saying you could get almost the same voltage drop with half the amount of silicon diodes.  

No two diodes only, back-to-back.  As I understand it the value of the diode determines HOW MUCH reduction you get- I got a LOT with the ones I used.  I have (just) ordered a bag of 

1X(30 PCS 400V 3A Polarized Rectifier Diodes IN5404 G9T4

30 pcs. for $ 1.42 from http://www.ebay.com/usr/hipsale on e-Bay.  I assume that they will work in similar applications, but if not- $ 1.42 down the drain.