A 5W resistor is much too large. However, if you have the typical square ceramic 5W resistor, there lurks inside a perfect 2W smoke resistor, you just need a vise and safety glasses.
I have tinkered with the smoke regulators, but I've only been about 1 of 3 in repairing them, it just wasn't worth the trouble. Now that Lionel had deep sixed them, it may be a useful exercise. Truthfully, I prefer getting rid of it as it's a trivial wiring exercise, and I never have to deal with the regulator again. They were always flaky, and a constant failure item.