Assuming this source is correct:

...they changed to a sintered metal casting process using an alloy that included graphite, nickel or chromium (actual recipes are not known, nor available from any source that I could find).

http://tincanbandit.blogspot.com/2014/02/winchester-model-94-rifles.html

Castings can generally be welded, provided you know what alloy they are made from and use a welding and pre/post heat treat process appropriate for that alloy.

Would be worth contacting Winchester, hopefully they can/will tell you the alloy they used so you can find an appropriate welding process.