Assuming its a Detroit....Harper (or i guess Wajax now), is probably the place to go. If its a Mercedes, you are pretty much stuck with Freightliner/Western Star.
I would not recommend a rebuild at most of the Freightliner dealers around southwestern Ontario, they really only shine at taking old pieces off and putting new pieces on under warranty. Sometimes they even replace the right pieces.
They don't shine at being economical.
IMO