chances are the seals need replacing in your brake master cylinder which is common for older bikes.
you can replace them or find a second hand master cylinder instead, or do what drewpy said
but the battery is 6v, so the bulb will only be recieving 6v not 12v, thus it is twice as big as it needs to be and will be half as bright.
well actually, taking into account ohms law it wont be EXACTLY half as bright, but you get the general idea
take out the bulbs and check to see if they are 6 or 12v, its not a foolproof way of telling what voltage it is but if the majority are 6v then its probably a 6v system. what year is the bike?
if you arent getting any output from the mag then this may be down to the reg/rec unit, check for an input voltage to this unit
the bearings are standard parts and available online, you can probably buy rebuild kits for your specific bike.
wrist pins usually come with the piston set so im not 100% sure you can get them seperately