Re: Translation help
Well, first someone will have to tell me the exact difference is between the English "motivation from within" and "self-motivation". As far as I can see, the former is a roundabout way of saying the latter. I suppose one might say that "self-motivation" is something done to oneself to motivate oneself, and thus lacks the sense of motivation that comes spontaneously from within oneself that "motivation from within" might carry. Nonetheless, 自発性 carries the latter nuance.
やる気 is basically "feeling like doing". Sure, in some contexts it could be used to translate "motivation" or "being motivated", but one can have やる気 without motivation (e.g., doing something fun just because you feel like it), or indeed motivation without やる気 (e.g., doing something because you know you should, even though you don't want to)..
To my ear, or 内からのやる気 sound odd because in as much as やる気 is a feeling or intention, it perforce comes from within. 自分からやる気を出す sounds better, more idiomatic, though I'd be inclined to say 自分で rather than 自分から, again because やる気 perforce comes 自分から, at least in this context.
But I think 自発性 works best because it means exactly what it says on the tin: 他からの影響・強制などではなく、自己の内部の原因によって行われること。 Without influence or compulsion from outside, something done originating from within oneself. I can kinda see Niall's objection in that it carries a sense of a personal characteristic, perhaps that some people have and others don't, and I think Niall's seeing the OP's phrase as referring to something that anyone can express. Where I would disagree with that is that 自発性 is something that can be worked on and developed. It can be encouraged by others, or oneself. We all have 自発性 -- it's just a question of whether it's dormant, or selectively expressed, or always expressed.
What we need, though, is not a native speaker (though those are always nice), but more context from the original poster. We cannot make bricks without clay.