Welcome to the campfire!
You would need a new stock for that early 1899A, plus a replacement buttplate, plus a lever. Can find the levers and buttplates listed on ebay or gunbroker occasionally, but finding a stock is near impossible.
A few places might be able to help you out with a replacement. Boses of Nebraska might have a stock template they could fit to your receiver, don't know what they charge. If you're a woodworker or somebody who learns quick and has a lot of patience, some other places on the net sell replacement stocks - but they'll take hours and hours of fitting.
Here is one of our member's 1899A's, so you can see what it looked like originally.