MTV video format conversion app?

I'm an old-school Visual Hub user when it comes to converting video formats. Unfortunately, that app is no longer being made and it doesn't solve my current problem so looking for some advice.

My nephew won an MP3/video player as part of a contest at school. Its not an iDevice but its a prize and he's enamored with it. The problem is, although it supports video, it only seems to support files formatted in the "MTV" format which I understand to be really limited technology. Hey...its a kids dream I'm working on here, so lets not go crazy with discussions on tech specs, etc. I'm just trying to help him out.

Can anyone recommend a decent media conversion program that will let me convert MP4's, AVI's and such to the MTV format? Its likely time I upgrade from Visual Hub anyways so I don't mind spending some money on a useful app. The problem is most of those I see mentioned in the forums here don't specifically mention the MTV format.

App advice please?