Yeah, I bet you were upset. I have never purchased a demilled gun, but I suppose they have to exclude a portion of the gun so you can't just weld it up and shoot it again. I don't know, but, that photo just breaks my heart! Seems to me they could have just pinned the cylinder and welded it closed or something.
Will the springer front end fit? My worry would be destroying something else, and then not having it fit. Like throwing the front end of a ford on a chevy or something, and now you have two things ruined. Is the springer metal, or plastic? I would not do it if it's plastic.
I would try getting a block of aluminum in the proper thickness, shaping it with my dremel so it matches the lines of the RG, drilling holes where needed, and then try to attach it. You might have to do a lot of metal work. Honestly, that could end up being a massive waste of time if it doesn't turn out right.
Tough call, that's not going to be easy fix/restoration though. But I will keep thinking on it for you and maybe we'll have some inspiration.