GRJ,
So step one is determine if the engine was PS1 or PS2; I have the box, just look at the original MTH number and/or MTH website. For sure the donor was GP9 (put an IC cab on it of course).
Step 2 is see if a fried triac is the problem. Could be a loose connection.
Step 3 is see if I can find a replacement and repair.
But Step 4, what do I do, put a resistor in series with it? Give up? Swap?
Most definite the donor engine was NOT PS3.
Then another project is lift a ERR Cruise out of a Williams Trainmaster and into a set of MTH PS1 NP F3, and as I recall, I need to buy some plastic shield, of which you already answered.