It's not obvious how to fix this broblem and, unfortunately, there is no quick and easy fix. In order to resolder the bush, the boiler has to be separated from the firebox, and this is made more difficult by the fact that the whistle is in the way!
First, you need to remove the rivets holding the engine frame to the firebox. In order to avoid the risk of drilling into the boiler, it's best to drill off the head of the rivet using a large drill, rather than drilling through the length of the rivet. (This is the way I prefer to do it anyway.)
When drilling the heads off, it's easiest to use a pillar drill but this is not essential. A hand drill will serve if you take care. Drill slowly and carefully untill the head comes free. For obvious reasons, don't go too far!
Once the frame is free, you can bend it back to gain access to the firebox top rivets. Drill the heads off these in the same way.
You also need to do the same on the two rivets holding the back end of the boiler to the firebox. There is no need to touch the rivets holding the gauge glass (if there is one).
At this stage, you have to bight the bullet and accept that you are going to loose the whistle! If it's the spring type then new ones are readily available. Cut off the bracket supporting the lever, using a junior hacksaw. Do this as close as you can get to the bush.
You should then be able to lift off the chromed part of the firebox over the whistle.
Saw off the whistle barrel as close as possible to the bush, but being carefull not to damage the bush. If the bush is very loose then you might need to hold the whistle in Mole grips (aka vice grips).
At this stage, you should be left with just the threaded portion of the whistle still in the bush but with clear access to the joint between bush and boiler.
You must then thoroughly clean the area around the bush using a wire brush.
If you have one of these fibreglass pencil brushes then all the better.
Having got rid of all the crud, wash liberally with a solvent (such as meths or IPA). You can now solder the bush into the boiler. Be EXTREMELY careful not to let the solder touch the remains of the whistle! For this sort of job, I prefer to use solder paste and a small pencil butane torch. Alternatively, you can use the type of solder used in electronics, available in small diameter and with built in flux. Either way, don't rush it. If you get solder on the inside of the bush then you've got problems!
Now that the bush is firmly soldered in place, you can remove the remains of the whistle. The easiest way to do this is by use of a bolt extractor. (Note the left hand thread.)
If it doesn't come out easily then you will need to soak the thread in release oil eg WD40. Do NOT use graphited release oil! This will make subsequent soldering impossible. If the new solder gives way then clean up the joint, resolder it and try again. You might have to do this 2 or 3 times if it's really tight.
Once it's out, then it's a good idea to redo the solder one last time, because the extraction will have stressed the joint. It should then be possible to remove the rest of the firebox and do a proper job of cleaning and repainting (if you so wish). If you don't want to repaint, then the damaged paint around the bush is not important, because it will be inside the firebox - visible through the ventilation holes but not obviously so.
Then, as they always say in the car servicing manuals, reassembling is the reverse of disassembly. Be sure to use the correct size rivets and, most important, for the two rivets going into the boiler, these must be self sealing copper ones.