I think I would be looking at buying a secondhand tailstock from another lathe of similar centre height and modifying the base to fit your machine. I can't see in the pics if it is flat ways bed or V ways, but would say a Myford tailstock be suitable for adaptation? V ways could be machined into the base if needed, or if the whole thing was a bit short, a piece of flat steel gauge plate etc bolted and doweled to the bottom to suit the bed.
Otherwise, yes it's possible to fabricate from steel. I made a GH Thomas design versatile dividing head this way. Rough machined the steel pieces to desired size, arc welded them together, heated the whole thing up red and buried it in lime overnight to anneal and stress relieve, then did all the final machining as you would on a normal casting. It worked fine.
A tailstock might not be too hard to fabricate. Turn a suitable sized cylinder for the quill to ride in, with an undersize hole bored up the middle, use a piece of 20mm flat plate for the base, the a couple two pieces of 10mm flat plate to make the sides that rise from base to barrel, set at an angle like an A frame, then a 10mm piece welded over each end of the A frame to form a tapered box structure. Then you have to have a furnace, or those lightweight firebricks and a large torch to heat it red hot after arc welding and slow cool.