HoModeler, who said anything about fitting it with a decoder

All the OP was asking about was how to get it moving again

If you can't get the bottom plate off even with all the screws you can find removed, check to see if it's glued on or if maybe the plastic melted over a screw (Yes, that can happen, and it is super-annoying).
I believe it was North West Short Line that offers wheelsets that can be put to this task, but don't buy until you've got the loco fully open and confirmed what's wrong.