It's not a simple solution, and it's basically impossible. Mercedes can't supply engines to five teams on the grid, including themselves, especially since they already want to reduce that number.
As I said at the beginning of the year, no other engine supplier wants to supply engines to Aston Martin, knowing they have Adrian Newey on the team. The only option for AM, besides Honda, with any chance of happening is Audi. However, the chances of Audi agreeing to supply engines are minimal.
Maybe Honda leaves F1, forcing Audi, under FIA rules, to supply engines to AM due to having the fewest customer teams. Or, with more luck, they might make an exception for Audi as a new manufacturer, shifting that responsibility to Ferrari.
