Just buy a replacement from Henry. They have excellent customer service department who will help you out. They actually list replacement stocks on their website. I don't see the one for your 45-70 but a call would probably locate one.
https://henrypride.com/collections/rifle-parts
I don't know of anyone making aftermarket stocks for them. Boyd's only makes one for the 17 HMR as far as I know but it may be worth calling their customer service.