The game is pretty easy to hack. Even I, a completely novice programmer, can hack it.
Ok:
High health:
Open server.cpp in the fpsgame directory in src (src/fpsgame/server.cpp)
And then do a search in the text for maxhealth
Eventually, if not at first, your search will take you to what value maxhealth is (maxhealth = 100)
Change that 100 to a higher number 
Recompile (make sure you remove the object file of the cpp file you just edited)
And you are done!
Unlimited ammo:
This is a hack that increases your ammo instead of decreasing it each time. So its not all weapons, but merely UNLIMITED AMMO
In the same directory as last time, open up weapon.h (or weapon.cpp, whichever one is present) (if you have both do the search in each one)
Scroolll down until you find the following line of code:
Code:
if(d->gunselect) d->ammo[d->gunselect]--
This handles shooting; each time, it decrases by one (hence --)
And change it to:
Code:
if(d->gunselect) d->ammo[d->gunselect]++
Instead of --, it is ++, so free ammo on every trigger press!!!
Do the rest yourself (that was the unlimited ammo hack)
And that's pretty much all I could do... For the smarties out there, you would know what do do if you want higher insta health or higher ammo, its close to the other hack variables.
NOTE: my health hack won't work on online games.
But the ammo one does!!!!
And health works offline