Forgive me if this is a holly grail frame I have never heard of, but... To me a frame that hit so hard it wreaked the fork, and split the main tube is a write-off. If you were on tour on Mars and your only option were to weld it up, you would clean away any rust first, and weld the tubes as well as possible or patch them. Tubes can be welded, some of my practice when starting out was to hacksaw a spit in tubes, and then weld it over, but there is a cost in tubing strength which is provided for in the joints by butts or generally thick walls, elsewhere it would be a weakness.

You likely have an alignment problem also.

Forks are easy to come by from any major parts supplier that deals with QBP.

It is possible to repair lugged frames by inserting new tubes, but it is a major job and it sounds like this frame went well past the yield point already.