Deka makes a 1.7lb & a 2.9lb battery w/o acid. I have been using the 2.9 lb battey which is 3 amp, 12V but the lighter one is only 1/2amp less. I removed about 60lbs so I lost the starter & stock battery so this batteies only function is to power the 4wd & lights so it doesn't need to be very big so I would think the 2.5amp version would work altough I never tried it. These are sealed so you can mount them anyway you want. Their number is 610-682-6361 at East Penn Mfg Co.
Jim Maloney 98 400 Scrambler

 

I saw an interesting setup on a flattrack Polaris one time . The owner had a suitcase style battery pack with heavy cables coming from it . He would plug into a recepticle he had the starter motor hooked to . The starter button was on the battery pack , once started he just unplugged it . Pretty slick , although he still had the weight of the starter there .

 

Yuasa also makes the 2.9 battery, and Bike master (a generic) does make the other size.