Bowser carries the former Cal-Scale line of detail parts. I'm pretty sure they have what you want. Good folks to deal with.
Precision Scale has the former Kemtron line as well as their own new detail parts. Again, I'm pretty sure you can find something suitable there.
MDC/Roundhouse steam loco kits - at least the ones with plastic boilers - typically came with several stacks in the kit. Perhaps you or a friend can glean one left over from building a kit.
Yes, you will have to remove the old stack. An advantage of a PSC or Bowser brass part is adding more weight to the locomotive, and in the right place - almost always a good thing for small locomotive models. You want as much weight as possible, and the model balanced between the drivers for maximum pulling power. 4-4-0 models tend to be heavy in the tail, and the spring tension of the pony truck doesn't help.
I'd love to see pictures when you get the conversion done. Could well be inspiration for me to do something similar. Right now, my Mantua General kit needs to go in the opposite direction - detailed to look like it was still in active use in the late 1890s.
hope this helps
Fred W
...modeling foggy coastal Oregon, where it's always 1900...
I have a Bachmann Spectrum 4-4-0 that came with the straight stack, though it included a 'wood' as well as 'coal' load for the tender. I'd like to convert the stack to a wood burners balloon or diamond stack. Where can I get these and how are they usually installed, that is.. does the straight stack have to be removed?
JaRRell