I know this has probably been asked before but I did a search and couldn't find anything.

After getting a new computer and losing all my 2003 stuff I am currently rebuilding my game from the ground up in RPGXP,

I am trying to convert the battle system but I am lost as to how stats work now

What is the difference between a peice of armor or a weapon increasing physical def/physical attack or just increasing dexterity or attack statistics.

my game has a very simple armor system, so if its the classic

DEX=Defense STR=Attack AGI=Speed and INT=Magic Power

and armor and what now just increased those statistics it would be perfect

And I just need my spells to be attached to INT, so what do I type into the skill slot for INT-F

This is all explained in the help file.  Try searching in the help file should get you all the algorithms the engine uses.

As for int-f and similar attribute, those will decide which stat should influence the effectiveness of the skill.  But as for the number, I cannot tell since it depends on your game, really.