That's a first for me, I have never seen the Kadet with a plastic cowl so wasn't sure what part you needed. If you posted a photo it would be easier but I will try. There are several ways to do it. If the cowl doesn't have cheeks or blisters, just a simple shape, I used to just seal all the holes with tape then smear a releasing agent {you can find it at boats shops} then I would just poor in dental stone or plaster paris, stuff a stick in the open end and when set the plug would slide out. If it had cheeks or blisters I would use tape and cover those then do the same thing. I have also made plugs by hand carving them from glued up balsa then after sanding them I would use epoxy resin over it and sand until smooth. I have used white foam and carved them too.
If you go into the scratch building forum you will be able to ask or do a search and get some better answers. When I make them out of foam I glass one up then just use gas or acetone to melt out the foam.
The real easy way is to phone SIG and ask if they have them in stock, if not you can check with Fiberglass Specialities and see if they have them. Last time I noticed SIG was still making the Kadets so they would still have them. FGS has them in real fiber glass though. Call and ask.