If you want to code the player's physics from scratch you have a few different options:
#1 Build it yourself
I've done this on a few occasions, this is one of the more.... difficult ways to go about it but it works and you have full control. Since pixels code is optimized you should have plenty of room to add what you want in the allotted space. I DID have a copy of the player physics in DouA format, but I'd suggest not using it because I modded the hell out of it and never kept a vanilla backup (oops).
#2 Use SIM's Player Size hack.
This is a bit more simple overall than rebuilding the code in x86 yourself, but is a lot more limited. You can change what frames are linked to what, like how many walk frames you have ect. but that's it really. If you're not much for code then check it out at the very least. It's in the latest version of Booster's Lab (I think) but if not you can get it in
Enlight's Haxinator folder.
#3 Take CSr/(cucky's)CSe code and run it in a .dll and make changes that way.
It will allow you to take the code in C instead of x86, but it's fairly technical to setup and I haven't actually done this method of code injection before. Once it's setup though it's the easiest to work with provided you know C instead of x86 instead.