drill a series of holes all the way round what u need to take out then take a 1/8 in course carbide cutter and connect the holes  then a good bastard file should do the job

Use the uncut cowling as a plug for a fibreglass mold and you can make as many as you want.
The Sterling kit cowl is too small in diameter for a scale 1/12th anyway, I used the Sterling Waco cowl and it's a little shy in diameter too.
You could even use the cowling as is to make a couple of fiberglass cowlings by just drooping them over your existing. Not too pretty but usable.
Chris...

If you haven't cut up your cowl .... make a fiberglass mold of it ..then make a fiberglass casting and use it and spare the original .... just in case you damage the cowl in some unforseen kissing the ground type of thing.. heaven for bid this happening.. but just in case. Then you can always offer a glass cowl for anyone who wants to scratch build the model. I have a Sterling Monocoupe  90 I'll do this for. Have the original cowl but would rather cut up a glass one instead.