It's fine, they've had enough time to replace you. However, why not offer to consult on any production issues that they have (at a reasonable hourly rate). As

It's fine, they've had enough time to replace you. However, why not offer to consult on any production issues that they have (at a reasonable hourly rate). As long as it's done in your own time, and it doesn't violate any non-compete you might have in general this shouldn't be an issue.