Looking at the old Jensen/Johnson 180 hp STC, (the easiest, simplest STC) they use the same nose bowl. New doors and new lower center cowl peice.
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I’m familiar with the original Dave Johnson conversions created for his glider towing operation. He used the original cowl but hammered out the bulges on the side doors. Not very pretty but effective. He also had trouble with overheating and had dual coolers mounted on some. He may have cleaned up the design in later renditions.
I used the Penn Yan kit for my cub. They provided a new standard sized, closed off nose cowl intended for using a small starter and rear oil cooler. The lower cowl and carburetor air box was made by Rubbert. (Very nice quality!)
The side cowl design was left to the installer. I made new side doors; rolled to get adequate engine clearance which ends up with 3 - 3 1/2” opening at the aft end compared to the original of 2 - 2.5 “
I soon realized that this opening resulted or contributed to a very cool running engine. Cylinder temps avg 360 and oil temp rarely above 160 without covering the cooler.
To the contrary I have a 150 horse cub with the exact same configuration but with the original tighter side cowl clearance and is a “warm”running engine.
I said all of this to recommend making new side cowls avoiding the temptation to bulge out the sides. They will look much better and may result in much better cooling.
