If AND ONLY IF the handlebars are structurally sound:

First clean off most of the rust with a brass wire brush then either:

1. User Rust Reformer to convert the rust to black phosphate and paint over it. Yes, you can get Chrome spray paint -- Rustoleum.

2. Remove the rest of the rust with rust remover, then apply Cold Galvanizing Compound (spray paint) -- Rustoleum.

However, since the down side is being killed in an accident if the handlebars fail, I would replace them if there is any hint that they have been significantly weakened.