ORIGINAL: GarySS
skylarkmk1 The thing that stopped me from doing that is the windshield shape. The Supercub has a squared off top as compared to the J3's more pinched off shape. That requires a different plastic windshield which I don't want to tackle in a scrach-build...molds, ovens and all... If an aftermarket windshield could be found and adapted to the Sig kit that would be a different solution. Reshaping the balsa wing areas to fit a different windshield would not be as hard as you would be working with wood and not plastic.
You shouldn't let so small a thing as a windshield stop you. Just get some material and make it yourself. It's the least difficult part of the kitbash. Take the J-3 windshield from the Sig kit and trim the "wing corners" off so you can flatten it out enough to trace a paper pattern, add whatever you need to bring it up to PA-18 specs, and trace it out onto another piece of clear stock. Trial fit the paper pattern until you have it just right. I think you'll find that you can make it flat and then fit it around the front corners of the wings. I did the same thing on my 1/5 cub when I built it as a L-4 (a Sig kit). I trimmed the corners off the windshield and made blocks from balsa to replace them for a more scale, realistic appearance.. Should be able to make the PA-18 mods as well.