Duke can be altered in a few ways if you desire. Maybe you want Duke to be able to have more health, or maybe you want him to move faster/slower. Maybe Duke is injured in your map and can't get above a certain health point and can't run as fast? Whatever the case, here's a few things to look at:
Let's start with altering Duke's health values..
Do a search for "MAXPLAYERHEALTH" and you should see the following:
define MAXPLAYERHEALTH 100
Also search for "MAXPLAYERATOMICHEALTH" which is close to the above line, you'll see this code:
define MAXPLAYERATOMICHEALTH 200 // doubles as maxarmor.
The first line defines the max health Duke can have when starting a level and through conventional health powerups, I would try to keep it between 1-999. The second line defines the max health Duke can have through Atomic Health powerups. Again, try to keep it between 1-999. Save your changes and test them out for yourself.
Here's another line that you can change to your liking - search for "RETRIEVEDISTANCE" and you will see:
define RETRIEVEDISTANCE 844
This code determines how far Duke can "stretch" to grab items.
This is where the "advanced" stuff starts. 844 is quite a random number and if you don't understand the reasons for the number it can be confusing. So, you have to do some guess work. Obviously, you know that "844" is the distance that the game calls for and that's the standard behavior. So try doubling the number to make Duke stretch further. You'll see that the higher you raise this number, the further out Duke will grab stuff. Get it too high, and Duke will collect items that aren't even in his line of sight! Lower it to 0 and Duke can't grab anything!
So now you know how to alter Duke's health and his ability to retrieve objects. Let's mess with two other things that you might find useful for your map project!
Search for "STARTARMORHEALTH" you will see this line:
define STARTARMORHEALTH 0
This code states that when Duke starts a level (pistol start), he starts with 0 armor. Let's say you've made a very challenging set of maps, but don't want to make the game easier by giving Duke higher amounts of health. Instead, just let Duke start out with armor when the game begins. This makes the level slightly easier, but not nearly as much as buffing up Duke's hit points. Change the code to this if you want Duke to have full armor:
define STARTARMORHEALTH 100
Alternatively, you could make it 50 or 75 to give Duke "Used Armor" instead.
Now, one other thing that you might want to alter about Duke would be his speed. Do a search for "RUNNINGSPEED" and you will find this line of code:
define RUNNINGSPEED 53200
define GRAVITATIONALCONSTANT 176
The first line controls Duke's speed, while the second line controls Duke's gravity. With the first line, lowering the number will make Duke move much slower, but decrease it too low and Duke won't move at all! Also, increasing this number of course makes Duke go faster, but you will discover that Duke's "Speed" isn't just about speed as much as it's about Duke's friction. Try raising it to just 63200 and see how hard it is to control Duke, bump it up to 73200 and Duke will almost always slide into the fence on the rooftop of E1L1 and kill himself, in the DOS game you'd receive an error instead. Personally, I would only add a little to the number, like 56600 if you want Duke to run more like Doomguy. I would lower it to 43000 if you want to "cripple" Duke into only being able to walk. Anything lower than 40000 tends to keep Duke from bobbing when walking, and he goes a little too slow if you get the number too low.
So the second line we mentioned earlier:
define GRAVITATIONALCONSTANT 176
This code changes the gravity constant in the game. Basically, the lower the number, the lighter Duke falls, the higher, the harder he falls. You'll notice that this code doesn't just change Duke's behavior, but it also makes the pipe bombs act differently as well as a lot of the other effects like blood splats and things that would "fall" to the ground. This is something you may be better off leaving alone, but if you are making a space themed episode this may be worth messing with in your case. When you increase the number Duke can't make a lot of the jumps he used to be able to make, and if you lower it then he floats down slower. So this is an effect you will want to test with your map.