With internals still in the case "shudders". I have never gotten a grinder close to a motor that was anything but fully stripped. Well there was this one time at band camp... Im not saying it won't work but why risk it? The other option is to use paraffin wax, you stuff some cloth into the crankcase and create a chip resistant layer with melted wax.
I don't actually have any cases that are un-ported to do a quick how to video with. I'll see if I can find a junker e50 case or if someone in the city needs a port match...
I port match my cases (and others) when I am in the process of building a motor. I pull all the internals out and give the case a good once over to get the oil and dirt off of it. Then I mount it in my vice and get to porting. After all the porting is done I bring it up stairs and give it a good bath in hot soapy water to get every last metal chip out.
A few other recommendations:
Full face mask https://www.amazon.com/dp/B000BO6RIE/">like this one is a great thing to have. The dremel is running at 25k rpm and tosses chips all over the place. It's kind of distracting to have em hitting you in the mouth when you are trying to see what you are doing.
A good light source makes life a hell of a lot easier too. Depending on how you need to hold the tool to get the cuts you want you might be casting a shadow across the piece and having a light that you can move around it a great asset.