If the tractor is a GT16, then it's a B43M, if it's a GT 18, then it's a B48M (18hp). I have a Sear with the B48M--it has approx. 400 hours on it and runs good. I think the engines are the same size so either way you could probably use it in a Case.
-----------Kenny, Any chance this is a B43M? The 446 Manual says mine would have been a BF-MS C19029---The Same?
One Caveat: Some of the Onan engines had external oil filters, and some didn't (My Sears doesn't)--this may come into play when swapping them out due to clearance issues, etc.