I just had my altenator changed in my 1996 mitsu rs and the mechanic told me its not charging but old altenator was fried and he said its the voltage regulator, theres other electrical probs too was wondering if i should just change the whole ecu and is voltage regulator a part of the ecu?

If the old alternator fails in a big way it would of blown the fuse, regulator is built into the alternator. this fuse can also affect the ecu.
120 amp fuse for the alternator and a 60 amp, with a meter on the live side of the alternator, connect a bulb to the feed and then the other end connected to the shell on a good earth. this will show you if the fuse is blown or not as the feed should go to the battery and as long as the battery has power it will light the bulb. or just use a meter and put on volts and do the same thing should show volts. if engine runs but you have problem check the earthing wire to the shell from the battery as this area can become a problem over time and will need cleaning back to metal and the connections will need cleaning. follow the earth wire from the battery to the shell and you will see the earthing cluster on the shell.