Having quite a bit of experience with antique cornets, I'll give you my opinion.....maybe 1 in 100 that you play will be anything you'd even
consider playing in public in a modern music setting. First, it was probably made to play in high pitch, so it won't easily tune to 440. Those turn of the century ones that have both high and low pitch bits are primarily high pitch instruments and play out of tune with themselves even when using the low-pitch bit and pulling the slides a little. The one you're looking at has no bit shown, so if it's missing, you'll need to have one made, which will cost about $40 if you're lucky.
Speaking of cost, there's a good chance the valves are worn and leaky, too, so there's another $300 repair bill. Once it's in good playing condition, you'll have to deal with problematic intonation, a tone you don't like (many horns from this era are brighter than you'd imagine), and a projection problem - they don't like to be played loudly.
Anyway, that's my take on it. If I were you, I'd find a nice modern cornet and try it with a proper cornet mouthpiece, maybe even an old one from the turn of the century. The mouthpiece will go a long way in getting that "antique sound" you're after, even on a newer cornet. I play a circa 1870 Henry Lehnert cornet with a Civil War band, and it's actually a pretty good playing horn when using the period mouthpiece. I've played 20 or 30 cornets made from the 1850's thru the 1910's, and this one is the best I've tried (and it's a looker
). I still wouldn't play it in a modern setting, though. My Bach 184G short cornet fills that requirement very nicely.
The guy on the video sounds good, though, doesn't he?
1870 H. Lehnert SARV cornet 