Mike,
Welcome to the fun of Shays. I know I have 5 of them in both Ho & HoN3, 1 built (2 truck standard gauge) / 1 under construction (3 truck standard gauge) & 3 in the closet. If you replace the gear tower gears you can get a descent running engine. The NWSL kit numbers are- 3-truck Bull Gear upgrade kit#188-6 / Partial re-gear kit#187-6 /
Re-gear kit# 186-6. I have used the 186-6 & 188-6 with good results so far.?
The kits I have are all square 4x4x12" red in colored box & one brown box. I bought the red boxed ones in the late 90's. The brown box was given to me in 2009.?
A word of advice if you can locate the book The MDC Shay Handbook : Author - Johnson : Publisher Oso
Then a good light,magnification visor, and patience.??