Vintage BMW Motorcycle Owners

I have a machine that I want to get on the road. I looked inside the tank and it has dried up crust form the old gas. i used a brush and vacuum cleaner to suck up most of the crud but I'd like to flush it out without harming the red coating or the excellent outside paint. Any thoughts?
Thanks. Frank. NYC.

What I have done with old tanks…which has rust, is fill tank have way with white vinegar with loose hardware (nut and screws)..put cap on and shake. Now with rust I would let sit over night and re-coat the tank with a sealant. But in your case I wouldn't use a lot of hardware and lightly shake and then drain…rinse with soap and water (dawn). The dry out by sitting it in the sun or air dry, then rinse out again with alcohol.

Oh..I forgot…I would also replace the petcock.