6 mm is .23622 inches. The hole is likely "ovalized" so ream (not drill!) the hole to a 0.25 inch round hole and put an inch rivet in it. Inch tubular rivets are widely available.

You will also need tools: Rivet squeezer (likely require a pneumatic device for a steel rivet) and the proper "roll" die. Local A&P may have the tools and install the rivet for you. It is a skill. Unlikely most people would get it right the first time. The tools are expensive.

I would and have done what UH suggested: Bolt, bushing, and locking nut, if space allows.