To answer the original question . . . you can use an antique flask. However, as hawg points out, you run the risk of damage to it over time. On the new ones, (I'm talking replicas of originals - not the CVS brass tube type) I've never been real happy with the tolerances of the gate as some of them allow for leakage in a hunting bag. It all depends on the quality of the flask though. If you go with a higher quality repro flask - such as a Dixon, (at least the ones that I've seen) the fit of the gate, etc. isn't too bad.
I had a number of both original horns and flasks that I picked up over the years that i liquidated several years ago. I had two original tasks that I used quite often - one was brass and the other zinc Both were tight and had tight valves. I was just careful in using them and didn't carry them when I was hunting. I also used several original horns - "day horns" that probably dated to the late 1800s to the early 1900s.
The zinc flask that I used that was original I picked up in the very early 60s. There weren't a whole lot of repros at that time - not like today. I chuckle when I see some of the listings on flea bay for "original antique flasks" which are obviously modern made replicas. As much as I hate to say it, I've always thought that things of this sort ought to be marked "replica" in some manner in a inconspicuous spot.
If a pair of '51 Navies were good enough for Billy Hickok, then a single Navy on my right hip is good enough for me . . . besides . . . I'm probably only half as good as he was anyways. Hiram's Rangers Badge #63