i have an enya 90 4cycle engine.my question is can you turn the engine upside down to mount it? i need to have exhaust down and i would have to mount upside down.

Yes, may be a bit harder to start, but you shouldn't have any real problems.

Jetts

You could mount it sideways to get the exhaust pointing down.

Aside from the fact that some engines don't like to run inverted, the main reason I don't like mounting them that way is that in case of a nose-over, the valve cover is the first thing to get nailed.