I think this has been asked some time ago although maybe with a slightly different twist to it. My own view is that I want all of my meagre collection to work and work as original spec if possible. So, like you, I will make a replacement part if I can or if an original replacement part is what I consider to be prohibitively expensive. With the gun working, would I use it all the time? It depends ... if I consider the gun to be 'fragile' then I might get worried about further malfunctions that I could not fix either due to my ability or to the cost. In this case I would most likely just fire the gun very occasionally to satisfy myself all is well. If I consider the gun to be bomb proof then I have no worries.
Interestingly I do not really count leather piston washers, breech seals or sometimes mainsprings as anyway 'collectable' so have no qualms or fears of making them if needed. And if, e.g. a self tapping screw has been bodged into a gun and destroyed the original threading, I have no problem with me retapping the hole to a size that will get the gun working and making/modifying a suitable screw to provide an aesthetically identical / pleasing appearance compared to the original.
I would always keep broken parts as a reference.
Cheers, Phil