First - of all that is a gorgeous little single seater. You MUST give us all the history, specs and performance if you have flown it. I looked it up, it is a RITER SPECIAL, serial 101 built in 1961!!!! At one time it was in AIR & MILITARY MUSEUM OF THE OZARKS. This is a classic. I would not change it. Recommend you fly it stock first and get CHT's from all 4 jugs before making cooling changes. My question is the cowl looks great. What is your motive to change it?
Second - Wheel Pants and Gear Leg Fairings - What I see of cowl, pretty, classic design, fairly efficient. I wouldn't screw with it. If you are trying to gain another 5 mph the wheel pants and gear leg fairings would be your first choice. I will repeat a few times... leave this old gal alone. It is so classic. It was frankly ahead of it's time. Nothing wrong with the wheel and gear fairings. It looks robust with better soft field gear and wheel fairings than RV's, which are made more for speed.
Third - Induction scoop and airbox - if you MUST screw with cowl start with the intake, install a VAN's style intake or FAB (filtered airbox) and scoop. This may not pick up speed but it will increase your MAP and HP in cruise. Also new exhaust, 4-into-1 or 4-into-4. I see "Y" pipes from the pictures, not as good for making the most HP. Increase HP increases speed. The stock inlet area is clearly a bit large, but design looks very good. The air intake lip looks sharp, but again my suggestion is leave it original like a classic car.... Looking at the picture it may already have an efficient Van's style airbox on it....
Fourth - Sealed solid plenum - Before you reduce the cowl inlet cooling area (fly with it stock and record CHT's) consider leaving the cowl alone and making a sealed solid plenum (verses soft seals against cowl). Many RV flyers have done this beautifully. This will pick up a few MPH and improve cooling. This will keep classic look and improve cooling. You can block off exit air if cooling is good and cut drag a little more. Love the look of the original cowl.
Fifth - Reduce cooling inlets area and improve design - Frankly this original design deserves to have classic original work, but if you must change the cowl you MUST (in my opinion) do the plenum at the same time. Just changing the shape, area of the cooling inlets will not help anything and may cause cooling issues. Why make a new cowl? Just modify this one.
Cowl Cooling Inlets - I would leave the cowl alone but if I was to change the inlets I would just mod the existing cowl. The topic of how and what to do is long. I assume this is a stock O320 or O360. About 24" sq inch for two inlets is about right (4" diameter round). Why round, well it has too advantages. It is easier to seal to the solid plenum with a circular soft duct that can be clamped down; and you want to move the inlets as far outboard as possible to get into clean prop are. Near the hub the air get's beat to death. However as I say in 4th item you can adapt the existing inlet shape to the solid sealed plenum, but it takes a little more creativity. It has been done before and done nicely by many RV builders....
Changing inlet areas to smaller size and making it more efficient will lower drag. KEY WORDS more efficient. How much does this change make? 5 mph with the plenum may be. It may make CHT's get a little warmer (may be), so I'd fly with existing set up and monitor all four CHT's first. Make sure a solid plenum is sealed to cowl inlets absolutely, a must. To just make smaller inlets and do nothing else is not a great idea. Poor inlet transition, poor pressure recovery, air leaks of inlet air to baffling is drag and loss of efficiency.
Typically ROUND is the way to go for simplicity and some aerodynamic reasons. Two 4" round inlets would be enough. You might go smaller but that is up to you. The inlet should have generous lip radius and the throat of inlet curved, like the top of an airfoil... where it diverges from front to back, gets larger towards the plenum. Round inlets would be MODERN on this beauty and look out of place. This plane has other drag issues, and at some point leave it and go fly. I think you should build an RV-3 or RV-4
If you are going hog wild you may want to add a prop extension. This will move prop out and allow you to make the cowl nose bowl more aerodynamic and allow room for good cooling inlet transition.
You could fly this and buy a used RV-3 or RV-4 fix-er-upper and experiment on that, but this old classic deserves to be preserved as is IMHO. Will going 5 or 10 MPH faster make you happier. It will still be slower than a stock RV-3 or RV-4. The RV-3 and RV-4 I'm also estimating will have shorter takeoff and landing distance and better climb rate. A cowl change will not make it an RV.