electrical question about my 85 250sx

[Trikotic]

is there any way i can rig up the electrical so the headlight can run right off the magneto? or do i have to have the battery. i am too damn cheap to buy a new gel-sealed battery. how long will a new gel-sealed battery last before i need to start puttin it on the battery tender? also, would another battery from like another bike work? whats the difference between 12v batteries that have different sizes? do they put out more power or what, cuz it was a long shot but i wanted to just try hookin up a larger motorcycle battery to my bike but i got nothing. thanks for the help

[dads]

if the alternator is working your light should work but will be dim.
any acid filled batt. that fits the batt holder will work. that is what i run, costs around $45.00 i think. got it at Auto Zone. they have a listing for 85 250 sx batt.