In order to gain access to the source you're undoubtedly required to sign a non-disclosure agreement and a non-compete clause.
That's the way it works in most of the software companies. But if you are good in negotiations, you can shorten the non-complete clause to 3 or 5 years then work on whatever you want.
BTW since MorphOS is not a legal entity, if you work on AmigaOS, working on MorphOS will not be considered as working on competitive clause.