You could use a battery eliminator instead of a battery.
http://www.ebay.com.au/itm/Battery-...25?pt=LH_DefaultDomain_15&hash=item58bc2fe9e5
What they are is a capacitor with a couple of fly lead's connected to them.
They store energy (12v) like a battery, but are 10x smaller, 10x lighter, it's only draw back is it doesn't have the same capacity.
But seeing as you don't have electric start, or need to power a radio on accessory's etc you don't need a battery.
So with a capacitor, with your engine running all your 12v accessories will still work fine, and you'll never need another one, they last for years and years
Instead of paying big $$ for one, you can make your own and just zip tie it to your frame or make a mount to suit your bike
Buy a good quality cap, and it will last.
Get something that is higher rated than you need, like a 100v, 10,000uF second bit is the capacity, so a 20,000uF will have twice the capacity.
You can get them with solder on terminals, screw/bolt on, and some have an 8mm bolt inbuilt into them to help mount it too.
http://www.ebay.com.au/itm/351275860035?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT