Kings and landed gentry did them every year.

Plenty of gunsmiths here can do that work. Kirk Merrington, James Flynn, Dewey Vicknair, there are lots of them. I doubt side lock guns need maintenance more than about every 5-10 years with modern lube and reasonable care from the owner. A wipe down after use and keeping an eye on where the screw slots are goes a long way.

Good luck.

Best,
Ted