Unless your current CRT is too dim to read, I wouldn't replace it with a used one. How do you know new (old) one is better? Since no one is making these anymore, chances are, it will not be that much better. LCD replacements are nice but cost is high. Since you have a nice 1GHz 4 ch scope, may be it's justifiable....
I have a different approach to re-capping. Quality parts Tektronics and HP used back then lasts longer than what people think. I've removed a few and tested them. Capacity and leakage are quite acceptable - most are like new spec. So when one fails, I'd replace. But otherwise, I leave the original one in.
About Dallas chip, there are new ones. (meaning freshly manufactured) Try looking at M48Z35Y and its family of chips. They are meant for Dallas replacement and they are made NEW. Once you program it, it engages battery inside. I don't know if there are types with real-time clock on it though. I've also Dremel'd and replaced battery inside. Only thing I do differently is I don't use sockets and coin cells. There are leaded types and they seem to have much better longevity spec. I'm sorry, I don't have part numbers handy but Digikey carries many. I certainly won't buy from eBay. Because of demand, there are so many fakes and new OLD stocks.