I have been looking at older Coleman stoves and lanterns lately on CL and Ebay. You can easily fined the older ones in good condition for very little cash. I found a 413D stove at a garage sale 22 years ago for $10. It was made in the early 1950's. After replacing the valve assy and the tank cap, it has worked well for those last 22 years of frequent use.
I recently discovered through the searching on Ebay, that parts are availabe there too. One thing I found is that I didn't need to replace the leaking tank cap or the leaking valve assembly. The seals are easily replacable. The tank gaskets can be found on Ebay from a guy who makes them out of nitrile rubber, I think. Couple bucks for 10. Instructions on how to replace them are found on the oldcolemanparts.com website. There are two sizes, so make sure you get the correct size for your model stove or lantern. My stoves and lanterns all use the same larger size.
If the valve is leaking, try tightening the nut a little. If that doesn't work, you need the valve packing. I bought a set of 10 from Coleman 10 years ago, and have only used one. Once again the oldcolemanparts.com has the info to replace.
If your older stove has a leather pump cup, keep it oiled and it will last a long time. Motor oil or 3-in-1 oil works. Pull it out and turn the cup inside out and soak in oil. The newer ones us a rubber cup that will wear out eventually, but still keep it oiled.
That website will take you through a complete teardown and rebuild, but it is good info for minor problems and disassembly info and theory of operation. It helps to know how they work.
I now have three stoves and three lanterns. I honestly prefer the older stuff, and I am always told how cool they are. I recently bought a used 424 dual-fuel stove for $30 so that I could burn pump gas on long trips, if I wanted. While it was made in the USA, it is definately more cheaply made. The steel case is thinner guage and was dented before I got it. There is less reinforcement of the case. The "feet" are formed into the bottom of the case, and are not adjustable like the old ones. The grill is not as securely attached to the case, just two points inserted into holes. The manifold had a leaking seam that the previous owner used something like JB Weld on. I will remove that and reweld eventually.
As others have said here, the old stuff will last forever with a little care and maintenance. If your stove is working, I would ignore the rust for the time being and just use it. If it doesn't work well, getting it running will be fairly easy and straightforward, and likely not expensive at all.