There are some great threads to be found on here about this.
I would probably follow Diggory Hadoke's process, which is to rub the stock down with ethanol and steel wool, raise any dents, lightly paper the whole thing (steering clear of steel parts), and then apply an oil finish until satisfied.
There are lots of opinions on the best finish. Mine is this: a 100 year old English gun will be most pleasing with a period and locale appropriate finish, which would be a 'slacum', the British gun trade word for an oil finish.
If you're feeling less ambitious, here is a good thread about cleaning up a grimey stock. I would start with traditional cleaner (linseed, vinegar, turps) then a coat or two of slacum (store bought or homemade).
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=503591&page=1
