Cheapest method and one which would retain the original parts: Are there any competent welders in your area that are skilled in G.T.A.W. aka TIG welding? The previous repair attempts with the solder failed because the metal was not bare and clean: It looks like big metal boogers, randomely stuck to both bolt and handle? The TIG process would be quite a "permanent" repair, where brazing or silver solder can at least be melted off to disassemble.
A gunsmith or competent machine shop could probably turn/mill a new bolt handle made from stronger metal than stock,(too avoid a repeat of the present problem) with oversized thread, leaving only the bolt thread to be tapped out to mate and the solder mess cleaned off.
Not sure how intent you are in retaining originality, it seems you are weighing between affordability and keeping a sentimental rifle in useful, but close to the condition you developed the attachment condition: I am sentimental almost to the point of OCD. I couldn't locate a shotgun bead for an old shotgun with sentimental value, I turned it on a drill out of Black Cherry when I was about 21 - I later turned one out of an old copper hydro wire, so when the wooden bead eventually breaks, I will have a replacement that will survive me.
