Howdy
Funny you should ask. On Friday I was going through my parts box to find a hammer spring that I could pop into an original Remington Model 1875 revolver. I bought this revolver a few months ago, and one of my projects was to get it ready to shoot in a CAS match. This involved taking it completely apart, cleaning it, removing all the old oil and grease, and lubing everything with Ballistol.
There was nothing wrong with the original spring except it is very stiff, and I wanted something a little bit lighter. I also wanted a different spring that would be a bit easier to install or remove than the original spring. Because of the way the 1875 spring fits into the frame, it was a bear putting it in or taking it out. I did not want to grind down the original spring, I wanted to find a replacement, so if I ever need to I can put the original spring back in. Or at least supply it.
Anyhoo, I had a bunch of springs that I assume were for the 1858 C&B Remingtons, and one of them fit the bill better than any of the others. I have no idea exactly where I got it, I am calling it a mystery spring. But it fit right in, and lightened the hammer pull a bit, just like I wanted.
I took it to the range yesterday to try it out for the first time. Don't criticize me too much for how big the groups are (note I was shooting at only 15 feet). This model was notorious for overly large chamber mouths. I was happy it was not keyholing and I got all the hits on the target. I took it to a match today and had a ball with it.
P.S. Regarding using modern parts in antiques, the question of metric vs inch parts is not relevant unless taking about screws and screw threads. Everything else you just have to try and make the parts fit the best you can.