I recently ran into a pickle with a frame; to install and remove the wheel, adding a half link to the chain was necessary, making the rear end just a tad too long for me. My solution was to modify the drive side dropout to allow the shorter chain, and enough room to take off the wheel without opening the chain.
The tools I used were: electric angle grinder (or a file if that's what you have), a black marker, a straight edge, a round object to draw a radius, and safety glasses.
I began by extending the lines of the dropouts, then following the curve. I guessed I'd need to extend the slot about 1/4", but because that was a guess, I removed just a little at a time, and tried to fit the wheel to avoid cutting too much.
I only modified the drive side dropout. I don't know if that is the best method, but I figured the less I did to the frame, the better.
I also went ahead and took some material off my adapter that sits in the axle slot, on one side where it meets the front of the opening. It allowed just that much less material to remove from the frame.
After all material was removed, in several stages, the final fitting of the wheel was complete. Had I taken any more out, the peg would have rested on the weld; no good.
The frame I have fits tight to the wheel as it is, but with a 1.9" tire there is still room. As you can see, the wheel sits straight and true even tho only one dropout was modified:
I only used an angle grinder for this, so it's not perfect. If you attempt this, be safe, and most importantly GO SLOW and remove/fit little by little. Good luck!