The idea is not theirs per se but any code or digital and physical assets may be. If you need those assets ask them for a release. If there are patents that you

The idea is not theirs per se but any code or digital and physical assets may be. If you need those assets ask them for a release. If there are patents that you can't work around then ask for a license. If you don't want a release, make sure your NDA has expired and don't use or look at the old code in question. You may want to hire a third party to write the code clean for you too. Or proceed as you wish and if you are later successful expect a lawsuit/settlement. Consider that the idea tax.