Help modifying a few things

I'm pretty new to ROM hacking, but there's something I really want to try with LttP.
I'd like to make a mage mode of sorts, where you start with the 1/2 magic upgrade and get 1/4 later in the game (which is in the game's data but unused if I'm remembering correctly). To balance that out Link would take more damage than usual. Things like the lantern doing a little bit of damage and higher magic refill drop rates would be nice too, but not necessary for it to work. I was wondering how I'd go about swapping these if it's reasonably doable.

where you start with the 1/2 magic upgrade

You will need a hex editor for that. Open rom in hex editor, remove header if it has one (200 bytes of the value 00 at the beginning). Than select "Go to" and type in hex address 27501. There is a byte 00 there (ie normal green magic). Change this to 01 (ie will take half green magic on game start). If you change it to 02, it will take quarter green magic from the beginning.

and get 1/4 later in the game

This was not yet documented, but should be doable with Geiger Snes debugger, since you need to trace the code and see on which location does it indeed swap the byte to 01 (regular upgrade by the Bat). So you thus need to change this to 02 on this new location.

To balance that out Link would take more damage than usual

This can be done with a special program Hyrule Add ons (should be on the site) by manipulating the Armor-1, -2 and -3 values. So basically you change the damage values for Armor-1 for instance to thus receive double damage from all enemies.

Things like the lantern doing a little bit of damage

This will need ASM obviously to also include the HP reduction when lantern is in used (along with magic reduction).

higher magic refill drop rates

This is actually the most difficult, since you have to trace each monster/sprite and find their drop rates, and then change some of the empty ones to magic jars (also need ASM tracing).

So all together quite orthodox problems need to be addressed for these goals to be done, which are not actually recomended for beginers, but you can stil give it a try.

Thanks for the reply. I figured the torch and drop rate edits would be difficult, but changing Link's defense values and what magic type he has seems like it's reasonable enough.
I'm not quite sure how to use the debugger, though. What function should I use for tracing the code?