yahama dragstar 125cc bike loosing power?

my brothers bike is the most unreliable thing going and usually ends up with me having to drive him somewhere after he can't get it going!

mechanically the bike is sound
electrically it isn't!

he got it with an aftermarket alarm that goes off if anyone touches the bike! this is a drain on the battery even if the bike is off!
now he likes it on it!
but he tends to be away alot (he lives a very free lifestyle) works freelance most of the time and you tend to not see him for maybe 2 weeks. as a result we have to drive the bike around for maybe a half an hour a day just to keep it alive! however it never starts up by itself it always lights up and makes a feeble attempt to power the starter motor the lights all dim and it gets weak so you have to push it in second and it usually starts up however if two days go by and noone has started it up the bike is so weak that nothing works.
he had the battery changed still didn't help!
is it the voltage regulator? the alarm? the bikes wiring? the alternator?
what should i check for?
thanks in advance!
 
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