How common is it for smiths to craft rare parts for firearms?
I can't speak for all smiths but in my case I've only done it a few times.
I think the main reasons for that, is the fact that I set my shop up to build custom 1911 pistols so I don't take on a lot of other type of gunsmithing, however being in a rural area I still do some.
The cost to fabricate a part from scratch is another reason.
I have fabricated a few parts for some opsolete old rifles but most of the time the owners will decide to just make wall hangers out of them instead of paying machine time and labor.
As for heat treating that can be a concern but many old guns did not have much done in the way of heat treating.
If the old part is still with the gun you can run a hardness test on the part.
If you are lucky and the part is broke with all the pieces intact, fabricating a new part may not be that difficult, don't confuse that with not being time consuming because many times they are.
Most of the time if you have to fabricate a part the part is missing and trying to find a blue print of the parts to get measurements from is impossible.
I don't believe I would try to keep a shop open just fabricating parts for old guns.
Get yourself a copy of machinery handbook, it has a wealth of knowledge on metal working, heat treating and tempering of metals.
I wish you the best in your endeavors.