So add a 4 then 0s until I'm at 6 digits? Perfect thanks I'll try that when I get home. I'll send you the mod link if you want it. WARNING I have very little mod completed, but I've added a few beta songs in the .exe
Well, as far as finding the lines [offsets I guess they call them], just remember that they are always 6 digits long, and always start with a 4. Then you just have to use ctrl-G to jump straight to that line. For instance, 0x032729 would mean 432729. The 0x at the first is irrelevant (it means hex, and you also might see $0 or just plain 00 at the first). If someone gave you an offset 0x0c3c0, then you'd go to 40c3c0.
Yeah, in fact, they're right next to the regular physics variables. There's a bit of stuff around 4156d7 where changing the numbers will change how fast you run, how high you jump, all that good stuff.
As far as easy ways go, you could mod the physics, and there are a number of "tweak values" in a bunch of the weapons that can radically change their behaviour with little-to-no effort. Umm, beyond that, there's NPC creation which is my favourite, if a little challenging, and engine modifications which are probably the most ambitious, such as GIR's 45 degree slopes. UI like the energy and health system I'd probably file under engine mods, but of course the difficulty always depends upon the desired outcome. The hardest part of it would probably be finding where it is and what it does, and then how to change it without breaking stuff.