Just depends how clean the incision was. I have had to mount a few that were cleaned before and they weren't sure if they would mount it at the time. Problem is when people generally clean birds they cut a little and rip away the skin from the breast. So you end up with many large tears along what should be a straight line. This leads to a lot of issues with ripping further and difficulty fleshing and tumbling prior to mounting. You may have very well sliced him open like a surgeon, from my experience the amount of extra sewing was substantial. Either way though, only way to know is get the taxidermist to see it first hand and make a call. Worse case is you avoid a belly showing pose.
It's not just a bird mount, it's a Schmitty Bird Mount